Advanced Lenses For Digital Cameras
The progress of digital SLR cameras is credited mainly to the trinity: lens, image sensor and digital image processor. At digishop.org, you have more than 1,350 digital SLR cameras to choose from. Selection of digital SLR camera is subject of many criteria, technical and personal. We do not touch the personal criteria; they are out of any discussion. In this article we use our professional expertise to bring into discussion several features of advanced lenses such as Image Stabilization – IS and Tilt Shift – TS. By explaining how these features work, we hope to help users to shoot better pictures. We strongly recommend you to consider this article as an incentive for reading more on the topics below and also for discussions and practicing on this subject. We review first some parameters frequently advertised for digital SLR cameras and also related to these advanced lenses.
Number of pixels such as 18MP and 22.3MP is frequently used feature for advertising digital SLR cameras. We show below the connection between the number of pixels and digital SLR camera lenses. Each digital SLR camera manufacturer has lenses compatible with its cameras. Lens compatibility covers not only the mount, but also optical parameters and electrical compatibility with camera body, including the operation of lens features. Obviously, there is a very strong functional link between digital SLR camera and the lens attached to it. If you do not use the lens to the recommended camera model, some features of the lens might not work, even if the lens and digital SLR come from the same manufacturer.
Camera format such as APS-C and Full Frame refers to the sensor size, as you see in the Cropped Images schematic for three very popular Canon image sensors, which shows also sensors dimensions.
Full Frame sensor size has the advantage of larger field of view of lenses designed for this format. The drawback is large size of camera body and of lenses designed for Full Frame format. APS-C sensors have some advantages for telephoto. Digital SLR cameras with APS-C sensor are more compact and lighter than Full Frame SLR cameras. Also the lenses designed for APS-C sensors are smaller and lighter than the lenses deigned for Full Frame sensor. It is easy to understand that optimization for a particular image sensor covers camera and also the lenses designed for that sensor. You will see below the main aspects below.
Some comments on sensor format, number of pixels, pixel area and pixel size for Canon and Nikon brands will clarify several aspects, directly applicable to other brands, too.
Canon APS-C sensor size is 22.2mmX14.8mm, or 329sq-mm [square-millimeters] area. For 18MP count such as in Canon EOS Rebel T3i 18MP camera, pixel area is 18.3sq-microns [square-microns], or about 4.3micronsX4.3microns size [1micron = 0.001mm]. Canon EOS 5D Mark III 22.3MP Full Frame camera has Full Frame sensor with 36mmX24mm size, or 864sq-mm, which is 2.63 times larger than APS-C sensor area. With its 22.3MP, pixel area is 38.74sq-microns or about 6.2micronsX6.2microns size, which is 2.11 times larger than the pixel area of APS-C sensor. From these examples, we conclude two aspects:
(i) One pixel of Full Frame sensor is about two times larger than one pixel of APS-C sensor and obviously captures about two times more light than one pixel of APS-C sensor. Therefore, Full Frame sensor is twice more sensitive to light than APS-C sensor, which is good for low-light exposure.
(ii) In all digital cameras, each pixel takes a sample of the image across its area. Entire digital picture is no more than a collection of samples taken by each pixel. Higher pixels per unit area or pixel density gives a sharper image than lower pixel density. In our situations, pixel density of 22.3MP Full Frame sensor is 0.026MP/sq-mm, and of 18MP APS-C sensor is 0.055MP/sq-mm. If the appropriate lens is used for each sensor, the math is quite simple if we consider APS-C sensor as reference: 2.63 times larger area of Full Frame sensor has 2.11 times larger pixel size. There is no significant difference in image sharpness between APS-C cameras and Full Frame cameras. Full Frame cameras have larger Field Of View or FOV, as you see in DX and FX Cameras with Lenses. We notice that DX cameras give slightly less sharp pictures, have lower light sensitivity and smaller size and weight than FX cameras. You can overcome light sensitivity issue by using a tripod, which however must be used for good quality picture in low light at ISO100 speed. Large FOV is the main advantage of Full Frame digital SLR, not considering other features not encountered in DX cameras.
(iii) Several lenses such as Canon EF-S 55-250mm f/4.0-5.6 IS II Telephoto Zoom Lens for Canon Digital SLR Cameras, are optimized for image projection on entire surface of APS-C sensor, having crop factor close to one. If you use Canon EF 75-300mm f/4-5.6 III Telephoto Zoom Lens for Canon SLR Cameras optimized for Full Frame sensor on camera with APS-C sensor, the image will be cropped with 1.6 factor in its central part, as you see in the Cropped Images schematic. However, APS-C cropped picture will reveal more details of the image than the picture shot with the same Full Frame lens mounted on Full Frame camera. In the schematic below, you can compare the blurred picture with sharp picture. You should be aware that blur was exaggerated on purpose, for understanding the difference. Spatial resolution is the professional term used for image sharpness: high spatial resolution reveals more details than low spatial resolution.
Nikon DX sensor size is 23.6mmX15.8mm, or 372.88sq-mm area. For its 24.2MP count such as in Nikon D3200 24.2MP CMOS Digital SLR, pixel area is 15.4sq-microns, and pixels density is 0.065MP/sq-mm. With Nikon 55-200mm f/4-5.6G ED IF AF-S DX VR Nikkor Zoom Lens optimized for DX sensor, crop factor does not limit the picture size. The FX sensor of Nikon D800E 36.3MP CMOS FX-Format Digital SLR Camera has 36mmX23.9mm size, with total area of 860.4sq-mm and pixels density of 0.042MP/sq-mm. Using DX sensor as reference, FX sensor pixel area is 1.5 times larger than the area of DX pixel with total area 2.3 larger than of DX total area. Looking at these numbers, at FX sensor we see 1.5 decrease in picture sharpness, but 2.3 time increase in light sensitivity. Not counting additional features of FX cameras, DX cameras give better picture sharpness with less light sensitivity than FX cameras. Using ISO100 and a tripod when shooting, light sensitivity can be compensated by longer exposure. With Nikon 35mm f/1.4G AF-S FX SWM Nikkor Lens for DSLR Cameras optimized for FX sensor, Nikon D800E DSLR will give theoretically finer details than Canon EOS 5D Mark III 22.3MP Full Frame camera with Full Frame Canon EF 75-300mm f/4-5.6 III Telephoto Zoom Lens. The picture file released by digital camera is subject to very complicate computations made by its digital image processor, which can affect details of the final picture.
DX and FX Cameras with Lenses schematic shows the fields of view of each FX and DX Nikon cameras equipped with their optimized lenses. DX configuration has a narrower field of view or FOV than FX configuration. Nikon D3200 24.2MP DX CMOS Digital SLR with FX optimized lens will crop the central part of the image with 1.5 crop factor, showing slightly finer details than the image of the same object taken with DX optimized lens.
Modulation Transfer Function or MTF is a measure of optical quality of a lens. Its graphs show the contrast and resolution of the lens from center toward its edge against a “perfect” lens that transmits 100% of the light and makes the image with infinitely small details. Of course, the perfect lens does not exist; it is just a theoretical reference. Any MTF graph plot shows on horizontal axis X, the distance from lens center toward its edge, in millimeters. The vertical axis Y shows the transmission from zero (no transmission) to 1 (100% transmission). MTF plots are traced along two conventional lines or directions named Sagittal and Meridional, for two spatial frequencies defined by two line patterns:
(sf1) Low spatial frequency MTF plots for 10lines/mm (100microns center to center line spacing) for lens contrast feature.
(sf2) High spatial frequency MTF plots for 30lines/mm (33.33microns center to center line spacing) for lens resolution or sharpness.
The schematic below gives better perception for line groups and also for Sagittal and Meridional lines.
For Canon APS-C image sensor with 18MP, there are about 23pixels within 100microns (10lines/mm) line spacing and 7.75pixels within 33.33microns (30lines/mm) line spacing. These numbers of pixels sample the image on image sensor above the Nyquist rate required for minimum recovery of the original image from sampled picture. As general rule, more pixels (samples) give sharper image.
Below you see two MTF examples for Canon EF-S 55-250mm f/4.0-5.6 IS II Telephoto Zoom Lens for Canon Digital SLR Cameras.
How can we read the above MTF diagrams? This is a zoom lens between F=55mm and F=250mm. Accordingly, the diagrams are for both focal distances F=55mm MTF Canon EF-S 55-250mm f/4-5.6 IS II and F=250mm MTF Canon EF-S 55-250mm f/4-5.6 IS II. Canon MTF Charts have specific meanings such as:
(a) Thick lines are for 10lines/mm, and thin lines are for 30 lines/mm.
(b) Black lines are for maximum aperture f/5.6 and blue lines are for f/8.
(c) Solid lines on the graphs are for meridional direction of the image.
(d) Dashed lines in the graphs are for sagital direction of the image.
(e) The difference between sagital and meridional graphs comes from lens astigmatism.
Conclusions from the MTF graphs:
(c1) The contrast is almost constant across the entire lens aperture for f/5.6 diaphragm, on both meridional and sagital directions. On sagital direction, for f/8, contrast drops monotonically up to 70% by the lens boundary.
(c2) The image resolution or sharpness has little change across both meridian directions for both f/5.6 and f/8 diaphragms. On sagital direction, sharpness has its maximum value in the central region of the lens up to 3mm radius, then decreases monotonically up to 40% by the lens boundary for both f/5.6 and f/8.0 diaphragms.
(c3) The contrast is almost constant for f/8.0 on meridional direction and has a small drop on sagital direction over more than 10mm radius. It is almost constant across the entire aperture for f/5.6 on meridional directions.
(c4) The sharpness has a drastic decrease for f/5.6 over 10mm radius, on both sagital and meridional directions. For f/8 on meridional direction is a 15% decrease from center to boundary, but for f/5.6 is a steep decrease up to 30% on both meridional and sagital directions. This lens gives a blurred image toward boundaries. Do not worry about this; it is very hard to notice that blur!
Lens contrast is linked to its resolution: when resolution decreases, contrast decreases, too; it is physics behind.
We show below also MTF plots for Nikon 55-200mm f/4-5.6G ED IF AF-S DX VR Nikkor Zoom lens, equivalent to the previous Canon lens.
Nikon shows MTF graphs also for minimum (Wide) and maximum (Tele) focal distances, with all notations on them. These plots show an overall lens behavior for Wide F=55mm focal length and for Tele F=200mm focal length.
There is no widely accepted standard for lens testing and for producing MTF graphs. Each lens manufacturer has its own procedures. Therefore, it is difficult to compare lenses with apparently similar focal distances and f/number from different manufacturers. However, the lens always works with the image sensor and with digital image controller. Ultimately, all these three elements must be considered together, or in other words, the picture counts.
Conclusions on lens optical quality:
(LQ1) Check to see if lens manufacturer provides MTF graphs. The lens should be made under tight quality control.
(LQ2) Select a lens with flat MTF graphs across maximum possible extent.
(LQ3) Final and the best check: look at the sample pictures made with your selected lens posted in your preferred sites dedicated to pictures.
We recommend several popular lenses. Each link below directs you not only to the lens, but also to many pictures made with that lens.
For each photographer, every type of lens leaves its “touch” on the image. This explains the large variety of available lenses compatible with practically all models of digital SLR camera from the same manufacturer such as Canon, Nikon, Sony, Olympus and Panasonic.
There are two types of subjects in photography: static and moving, and also two ways of shooting: handheld and using tripod. In all the above discussions, we assumed shooting static subjects with camera locked on rigid tripod. Very often, professional photographers shoot static subjects with tripod and at low speed such as ISO100 or ISO200 for the best picture sharpness. Assuming a perfect focus in handheld shooting, the camera always moves slightly during the exposure; the only question is how much it moves. If the camera moves within the sharpness limits of the lens, you do not see the movement in the picture. If the camera moves beyond the sharpness limits, the picture appears blurred more or less. The photographers use various approaches to get sharp pictures when shooting with handheld camera:
(sh1) Increase camera sensitivity to ISO200 or more. According to several camera specifications, you can shoot pictures with correct exposure up to ISO6400 or more. Be aware that at high ISO_speed, the picture gets blurred because of image sensor noise. Higher ISO_speed, more noise in the picture.
(sh2) Decrease the exposure time. For shutter speed of 1/125 or less, you get a reasonable sharp picture with handheld shooting. It is expected a blurred image when shutter speed is longer than f/[focal length]. This empirical rule shows that shooting with telephoto lenses are likely to blur the image for handheld shooting, unless the image move is mitigated up to the point where the camera autofocus can work properly.
(sh3) Open more the diaphragm or iris, to get more light on the image sensor. Be aware that the diaphragm number defines the depth of field – DOF of the lens. As an example, f/4 diaphragm has shorter DOF than f/8 diaphragm; therefore you are playing with the depth limits of image sharpness.
Image Stabilization (IS) feature mitigates the image shake on image sensor, thus allowing stable image up to four stops slower. This is a tremendous advantage versus the regular lenses without IS, especially when shooting in darker ambient light. Notice that this article is not an advertising for manufacturers of IS lenses. We provide professional expertise for the benefit of photographers by explaining how IS features work. All the examples have references to their respective sources.
The schematic Canon Conventional IS – OFF shows two arbitrary situations encountered in normal operation when using regular lenses for shooting, or when using a lens with disabled IS feature. This schematic shows a simulated shake for better understanding of the real case when using telephoto lenses. You notice the difference in scene positions between the two instances. The scene moves too much and too fast, preventing the camera to focus on the scene.
The schematic Camera Shake below explains camera movement during shooting.
When shooting either a static or a slow-moving scene, camera shaking in arbitrary way can be decomposed in tilt and shift shakes as shown in Camera Shake schematic above. Any tilt has a horizontal component (yaw) and also a vertical component (pitch) as figured out by the arrow pairs. Accordingly, there is an angular velocity sensor for each tilt component. Angular tilt of the camera lens produces an image shift on the image sensor. Camera processor uses the signals from these two angular velocity sensors for shifting the Image Stabilizer perpendicular on optical axis. Tilt Compensation schematic below shows how conventional IS works. You can see also the look of some Canon IS units.
IS operation is simple: when the lens tilts downwards, the image projected by the lens on the image sensor shifts. The Image Stabilizer Unit within the lens shifts its Image Stabilizer Lens perpendicular to the optical axis according to signals provided by both angular velocity sensors. It is obvious that the Image Stabilizer Lens moves simultaneously in two directions within a plane perpendicular on the optical axis. This reduces significantly the image shift on the image sensor up to the equivalent of four stops slower. This classical image stabilization has different brand names. Canon calls it Image Stabilization IS Mode 1 and Nikon calls it Vibration Reduction VR. It is really great help for many situations. Of course, IS or VR feature can be enabled and disabled anytime by the user.It is obvious that there is a time constant associated to this image stabilization.
WARNING: Long focal length is likely to produce blur with classical lens even when shooting at 1/125 or less. Image stabilization does not work if the image shakes too fast. When shooting with IS enabled, be careful to stay within maximum four stops slower than when IS is disabled.
The schematics below show two situations of aiming the scene with IS disabled and enabled. It is easy to notice how small is the image shift on image sensor surface with IS enabled.
In the above example, notice that Canon IS I works better on the swimmer than on the water waves and drops, which move faster than the swimmer. This example is in good agreement with the initial assumption of shooting either a static or a slow-moving scene mentioned at the beginning of description of IS operation.
When panning camera on a moving subject such as shown in Canon Image Stabilization IS-Mode 2 Panning schematic, IS operation may interfere with subject background. Both the subject and the background are moving.
In horizontal panning, the photographer follows the subject horizontally. The subject moves slower than the background, with sudden changes in its positions towards the frame. In this case, IS must compensate only for vertical changes. The background appears blurred.
In vertical panning, IS must compensate only for horizontal changes. Canon calls IS Mode 2, image stabilization on panning.
Schematic below shows Canon EF 70-300mm f/4-5.6 IS USM Lens for Canon EOS SLR Cameras with the operating instructions for selecting IS 1 and IS 2 modes of operation.
We recommend below several Canon lenses with IS feature. For the entire range of Canon IS lenses, visit the link Canon Image Stabilization Lenses.
(ISL1) Canon EF-S 55-250mm f/4.0-5.6 IS II Telephoto Zoom Lens for Canon Digital SLR Cameras
(ISL2) Canon EF 75-300mm f/4-5.6 III Telephoto Zoom Lens for Canon SLR Cameras
(ISL3) Canon EF 75-300mm f/4-5.6 III USM Telephoto Zoom Lens for Canon SLR Cameras
(ISL4) CANON EF 75-300 III F4-5.6
(ISL5) Canon EF 70-300mm f/4-5.6 IS USM Lens for Canon EOS SLR Cameras
(ISL6) Canon EF 28-135mm f/3.5-5.6 IS USM Standard Zoom Lens for Canon SLR Cameras
(ISL7) Canon EF 24-105mm f/4 L IS USM Lens for Canon EOS SLR Cameras.
Several Canon lenses have IS Mode 3 feature which stabilizes image only when the shutter is fully pressed. IS Mode 3 is recommended for fast acting scenes, such as sport subjects. IS Mode 3 is an extension of IS Mode 2, which stabilizes the image on direction perpendicular to panning.
We recommend several Canon lenses with IS Mode 3, such as:
(ISL8) Canon EF 300mm f/2.8L IS USM II Super Telephoto Lens for Canon EOS SLR Cameras
(ISL9) Canon EF 400mm f/2.8L IS USM II Super Telephoto Lens for Canon EOS SLR Cameras
(ISL10) Canon EF 500mm f/4L IS II USM Lens
(ISL11) Canon EF 600mm f/4L IS II USM Lens.
Hybrid Image Stabilization stabilizes for both tilt shake and shift shake. This feature is encountered at Canon EF 100mm f/2.8L IS USM Macro Lens for Canon Digital SLR Cameras. The schematic below shows the stabilization features of the lens and a picture of it.
Dynamic IS feature added on top of IS feature adds image sharpness when the subject moves either toward camera or in opposite direction, shown as Axial Shake in Camera Shake schematic. Canon EF-S 18-135mm f/3.5-5.6 IS STM Lens has Dynamic IS feature, very useful for video shooting. Below you have an image of this lens.
Nikon Vibration Reduction – VR technology uses the same principle as Canon Image Stabilization – IS technology, explained in Tilt Compensation Schematic. As you see in this schematic, Vibration Reduction Units and Image Stabilization Units look similar, but of course there are different, built according to each manufacturer’s system design: digital camera and lenses for them. VR II option operates faster than VR option.
For panning, VR detects camera movement and automatically suppresses the blur-correction function. If the camera shakes horizontally, VR reduces blur in the vertical direction. With this function, the panning effect is maximized. Panning Detection is effective regardless of the camera’s orientation or direction of motion, either horizontal or vertical.
A schematic of Nikon 18-200mm f/3.5-5.6G AF-S ED VR II Nikkor Telephoto Zoom Lens for Nikon DX-Format Digital SLR Cameras and a schematic of Vibration Reduction features of VR and VR II models are shown below.
We recommend below several popular models of Nikon lenses with VR features, from the top list of our Nikon lenses collection. For our entire VR collection, visit Nikon VR Lenses for Nikon digital SLR cameras.
(VR1) Nikon 55-200mm f/4-5.6G ED IF AF-S DX VR [Vibration Reduction] Nikkor Zoom Lens
(VR2) Nikon 18-200mm f/3.5-5.6G AF-S ED VR II Nikkor Telephoto Zoom Lens for Nikon DX-Format Digital SLR Cameras
(VR3) Nikon 55-300mm f/4.5-5.6G ED VR AF-S DX Nikkor Zoom Lens for Nikon Digital SLR
(VR4) Nikon 70-300mm f/4.5-5.6G ED IF AF-S VR Nikkor Zoom Lens for Nikon Digital SLR Cameras
(VR5) Nikon 18-200mm f/3.5-5.6 G ED-IF AF-S VR DX Zoom Nikkor Lens
(VR6) Nikon 18-200mm f/3.5-5.6G AF-S ED VR II Nikkor Telephoto Zoom Lens for Nikon DX-Format Digital SLR Cameras
You see sample images with Nikon VR disabled (OFF) and enabled (ON).
Tilt Shift – TS Lenses
In the schematic Perspective Correction with TS Lens the same building was shot from the same position with a regular wide angle lens (left) and with TS lens (right). The picture on the left has the inherent geometric distortions of the wide angle lens when shooting relatively close to subject. Skilled photographers use sometimes these distortions for producing special effects.
When you want the building to appear straight all the way up, the use of Tilt Shift – TS lens is required. The operation of TS lens is based on Scheimpflug principle shown in the schematic Tilt For Focus on Oblique Subjects below.
Scheimpflug discovered more than 100 years ago that you can get sharp images with very large Depth Of Field for subjects making an acute angle with the image senor (in our case), if you tilt the lens in such a way that the subject plane, the lens plane and the image sensor plane all meet along the same Hinge Line, followed by a lens shift to make the image on the sensor. The schematic Tilt For Focus on Oblique Subjects shows the Hinge Point, which is the intersection between the Hinge Line and the plane of schematic.
Practically, with TS lenses you make perspective corrections in two steps:
First, tilt the lens for focusing on subject.
Next, shift the lens until you get the best perspective correction, as in the diagram below.
If necessary, make another iteration until you get the best image, then shoot.
Electro-Magnetic Diaphragm – EMD module of canon works in tandem with electronic lens system for providing accurate control of light flux on the image sensor and almost circular aperture shape. The schematic Canon EMD below shows simulated extreme positions of diaphragm.
Below you see a picture of Canon TS-E 17mm f/4L UD Aspherical Ultra Wide Tilt-Shift Lens for Canon Digital SLR Cameras.
You can have access to our entire Canon TS lenses collection by visiting the link Canon Tilt Shift Lenses. We recommend below from the most popular models:
(CTSL 1) Canon TS-E 24mm f/3.5L II Ultra Wide Tilt-Shift Lens for Canon Digital SLR Cameras
(CTSL 2) Canon TS-E 17mm f/4L UD Aspherical Ultra Wide Tilt-Shift Lens for Canon Digital SLR Cameras
(CTSL 3) Canon TS-E 24mm f/3.5L Tilt Shift Lens for Canon SLR Cameras.
Follow the link Nikon Tilt Shift Lenses for our entire Nikon TS Lenses collection. We recommend several models below:
(NTSL 1) Nikon 45mm f/2.8 Perspective Control-E Nikkor Aspherical Manual Focus Lens – Grey Market
(NTSL 2) Nikon 24mm f/3.5D ED PC-E Nikkor Ultra-Wide Angle Lens for Nikon DSLR Cameras
(NTSL 3) Nikon 85mm f/2.8D PC-E Micro Nikkor Lens.
We suggest also Arsat brand of TS lenses for Nikon, Canon, Sony and Minolta cameras.
Fisheye and Fisheye Zoom Lenses
These lenses create special effects even during shooting. Of course, the enthusiast is limited only by imagination for post processing using photo editors available in our site in Digital Lab department.
You see below Canon EF 8-15mm f/4L Fisheye USM Ultra-Wide Zoom Lens for Canon EOS SLR Cameras and two pictures taken with this zoom lens.
We recommend also the popular Canon EF 15mm f/2.8 Fisheye Lens for Canon SLR Cameras with fixed focal length. The schematic below has the picture of the lens and also two sample images made with it.
We recommend Rokinon FE8M-N 8mm F3.5 Fisheye Lens for Nikon (Black), a very popular lens. See below the lens picture and two sample images taken with this lens.
We highlighted some advanced aspects of lenses for digital SLR such as MTF, image stabilization, TS and fisheye. For better understanding of lens features, we explained them in connection to several parameters of image sensors. Our idea is to help the photographers to take a better decision when buying lenses with advanced features and also for using these lenses at their full potential by knowing the basics behind their operation. Obviously, our purpose was not to exhaust the subject, but to reveal several important aspects. We hope that this article will help the photographers for better use of presented lenses and also will stimulate their interest to go in further details.