The Digital Sensor: A Guide to Understanding Digital Cameras
by Wesley Fink on April 21, 2008 1:00 AM EST- Posted in
- Digital Camera
Lens Equivalence
It is important to understand that a 50mm lens is always a 50mm lens, as that is the focal length. That 50mm specification affects depth-of-field and other image characteristics tied to the lens focal length. However, we can calculate focal lengths for each multiplier that will give the same field of view in the finished image.
With a closer look at field of view and the impact of the changes in field of view, it is easier to understand recent DSLR lens developments. Early DSLR lenses were generally 35mm lenses mounted on the new smaller sensor cameras, except for complete new systems such as the 4/3 digital-only system championed by Olympus.
Using 35mm or full frame lenses was great if your primary interest was telephoto and bird photography, as that 35mm 70-300mm lens that was the second lens for most film buyers now had a field of view like a 112mm-480mm zoom on the new Canon Digital Rebel. Unfortunately interiors, architectural photography, scenes, and fans of the extreme wide angle point of view were left in the cold in the early transition to small digital sensors.
That has been corrected in recent years with lenses designed for smaller sensors, lenses like the Canon 10-22mm, the Nikon 12-24mm, and similar APS C zooms from Sigma, Tamron, and Tokina. Today, whatever your mount and lens multiplier, there are lens choices that can cover the full range of choices for field of view.
A few lenses by Sigma are actually available in every mount and multiplier listed above. Obviously, these few lenses were originally full-frame 35mm that have been carried over with new coatings for improved performance and reduced flare on digital sensor SLRs. One such lens is the Sigma 24mm f1.8. The field of view on the different mounts and sensors this lens will fit illustrates just how the digital sensor size can influence the use of any lens. On the full-frame Canon5D, IDs III, and Nikon D3 this lens is a fast super wide 24mm. On the Canon 1.3X pro models it is still a fast f1.8, but with the FOV of a 31mm moderate wide angle.
On the Nikon D300/D60, the Sony A700/A350, and the Pentax K20D/K200D this fast lens is now a moderate wide angle to near normal lens that shoots images with a 36mm angle of view. On the Canon XSi and 40D we are at 38mm, which most would consider near normal. The Sigma SD14 FOV of 41mm has definitely crept into the normal range. Finally the 4/3 mount version of this lens is one of the "normal" lens choices on the Olympus E3/E510/E410 and Panasonic and Leica 4/3 digital SLR cameras. The 24mm on a 4/3 camera looks at the world as though it is a 48mm f1.8 lens, and competes with the Leica 25mm f1.4 as a much lower cost normal lens.
Similar comparisons could be made in other focal length ranges, but you get the point. Olympus makes a 70-300mm telephoto lens for 4/3, and it is much sought after by "birders", because the view on a 4/3 camera with this lens extends from 140mm to 600mm.
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Johnmcl7 - Monday, April 21, 2008 - link
I do agree with you there that it is desirable to have deeper depth of field (particularly on macro) but I think the difference is that on an SLR you have the choice - you can shoot wide open and have shallow depth of field or you can stop down (and normally have to use up the ISO advantage) to get depth of field. On a camera with a small sensor you are generally stuck with the deep depth of field whether you want it or not.John
finbarqs - Monday, April 21, 2008 - link
I hope nobody is using F/1.2 when shooting a portrait. Unless you want only a nose to be in focus. With something THAT shallow, you either have to REALLY step back and use distance to your advantage to get what you want in Focus. Professional Wedding photographers will not TOUCH anything above f/5.6 as you don't want to get ONLY the bride in focus while the groom is OOF.People, remember there are multiple factors that affect DOF. Aperture is definitely one of them, distance, and Focal Length.
Obviously you want something very shallow with great bokeh, then you would get close to your subject, open up your aperture, and have a telephoto focal length.
The thing is, on a FF sensor, you "see more" on the same focal length as a cropped sensor. Well put wesley, a 50mm is a 50mm. For those of us with cropped sensors, slap a 50mm on your camera. (or zoom to 50mm). Look at your subject with your naked eye, then look through the view finder. SAME SIZE AND SHAPE! Nothing gets larger! Supposedly a telephoto range (greater than normal) should have some zoom effets shouldn't it? I mean APS-C sensors are either 1.5 or 1.6 smaller than a full frame, that means the normal focal length should be around 27mm? So 50mm should be 75mm for nikons and 80mm for canons! That means you should see some zoom in 50mm! but nope! you don't! in fact, the only time you'll see the zoom is AFTER you fire the shutter and it displays the image to fill that 2.5" or 3.0" inch lcd you have! (or even 2.0") So where does the FF excel? you place 2 cameras exactly side by side of each other, same focal length, same focus point. Same lens. Through the Viewfinder, you'll see the nice Bokeh of your subject on BOTH cameras, exactly the same. Difference? the FF has more frame coverage! Thus, having better "DOF" (or lack thereof) because of the more frame coverage! On this note, let's all by Hasselblad's Medium FF and get some nice digital backs! ;)
haplo602 - Tuesday, April 22, 2008 - link
Hmm ... In my understanding the difference in FF to APS-C is CROP.A 50mm lens on APS-C has the same DOF, same perspective characteristic as on a FF camera, but the sensor is smaller, so you basicaly get a CROP of the original FF image. There is only a perceived zoom effect as the image is a smaller area.
For Nikon you get 1.5x smaller image area, but everything else is the same as on FF.
melgross - Tuesday, April 22, 2008 - link
It has to do with distance from the subject as well. Perspective is the ratio of the distance to the subject to the distance to the background.So, to get the same size on sensor, with the same lens length, you must either move closer, or further, changing the perspective, and the depth of field.
finbarqs - Monday, April 21, 2008 - link
Medium Format* sorry, typo :)Johnmcl7 - Monday, April 21, 2008 - link
Other way round - a larger sensor gives you *less* depth of field which as you quite rightly point out is a strong advantage of a larger sensor.John
pinto4402 - Monday, April 21, 2008 - link
DOF is also a function of the lens. Some ultra-serious photographers are willing to pay $2500 for a Canon 85mm 1.2L over a 85mm 1.8 (which is just as sharp and which only retails for $300), because the 1.2L has a softer, more pleasing DOF effect.TETRONG - Monday, April 21, 2008 - link
Cool article.Looking to buy a camera pretty soon here.
What do you folks think about the G9?
Is there a particular brand I should favor?
Also, are there any cameras available that do "tricky" stuff, like Infrared, Macro, or that neat effect when cities look miniature? (Can't find an example)
I read an article about a guy a few years back that would setup a scene, then he would walk around selectively lighting certain aspects in trippy ways to make a final composite image.
Anyone know what this is called, or where I can learn more about it?
Much obliged!
strikeback03 - Tuesday, April 22, 2008 - link
Is "fisheye" what you are looking for for the miniature cities?and the selective lighting is often called "Painting with light"
plenty of compact cameras can do OK macro if you can get really close to your subject, but as this is often hard for reasons of lighting and/or scaring off the subject, an SLR with a real macro lens is still beneficial.
pinto4402 - Monday, April 21, 2008 - link
There was an article at http://www.popphoto.com/popularphotographyfeatures...">http://www.popphoto.com/popularphotogra.../4923/th...about infrared photography. It also discusses a company that is modifying certain digital cameras for infrared use.