35mm SLR Lenses on DSLR's & DSLR Specific Lenses

Message boards on the internet are overflowing with misinformation about the SLR/DSLR lens debate. In the next little while, I will attempt to set the record straight by dispelling some common myths and misconceptions, and give you the straight dope on what really is and isn't. So sit back, relax, and clear your mind of everything you've heard so far about the subject, and let's get started.

First and foremost, I want to clear up the confusion caused regarding lens focal lengths, most notably those 35mm SLR lenses used on DSLR's with smaller APS sized sensors. Like I said in the introduction, forget everything you've heard so far so you can absorb the truth. Putting a 35mm lens on DSLR with an APS sized sensor does NOT change the zoom ratio. For example, a 24-85mm zoom lens will always be a 24-85mm zoom lens no matter what camera it's on. When it's on the store shelf, not attached to any camera at all, what is it? Right, it's a 24-85mm zoom lens. Congratulations, now that you've got that down we can move on to the next step. Let us assume that we take the same 24-85mm zoom lens and attach it to a DSLR with a 1.5x ratio compared to 35mm. Here's where people get confused and start believing all that misinformation floating around. The lens does NOT become a 36-127.5mm lens. Go back and read that last sentence again and commit it to memory. What happens is not a zoom ratio change, only a change in the crop factor. The crop factor for 35mm will be what we consider the standard or 1x ratio, and as stated earlier, the DSLR will have the 1.5x ratio. Once again, this is NOT a zoom ratio, only a crop factor. The smaller DSLR sensor will crop out the outside edges of the scene that would normally fall on 35mm film. This is why most all amateurs and even some unenlightened pros think it changes the zoom ratio of the lens. It essentially makes the subject look like it was shot with longer lens. Let us say for instance that we shoot a subject with the lens set to 24mm. On 35mm with the 1x crop ratio, the same image taken with the same lens setting on a DSLR with a 1.5x crop factor will almost appear as if it were taken with a 36mm lens. I say "almost" because it is not exactly the same. The only thing in the picture that has changed is the crop factor.

The perspective distortion and background compression of subjects in the picture will remain that of a 24mm lens and not a 36mm, if taken from the exact same position. If you move the digital camera back further from the subject to make the image on the DLSR sensor the same as what is on the 35mm film frame, the perspective distortion will stay the same, however the background compression will change slightly due to the change in subject to camera distance, just not as much of a change as would be encountered with a change in focal length settings on the lens. Okay, let's explore this in just a little more detail. A 50mm lens on a 35mm camera is called a normal lens because for the most part, what you see in the viewfinder is the same as what the average human eye sees. Go to a wider lens such as 35mm or 24mm and the perspective changes. Objects behind the subject appear further away from the subject that what they actually are, and object in front of the subject (or protruding from the subject for that matter, such as a nose) appear closer than normal. Just the opposite for telephoto lenses, which tend to compress or flatten the image. Take for example a 200mm lens, object behind the subject appear larger and closer to the subject that what they actually are, and object in front of the subject appear to be closer to the subject than what they are. It doesn't matter whether the lens is on a 35mm film camera or a DSLR with a 1.5x crop ratio. This is important to remember if you want your pictures to turn out the way you had planned. Mistaking a crop factor for a change in focal length like so many misinformed people do will cause your pictures to come out differently that what you might have planned for.

It's that simple! Now that you know the way your lens really works when put on a DSLR with a smaller than 35mm sensor, you can just smile and laugh when someone tries to tell you that your lens actual focal length changes.

Next let's explore whether or not you actually need that special DX lens for your DSLR. Lens makers seem to be pushing DSLR specific lenses for use on APS sensor sized DSLR's only. These digital specific lenses will not work with a 35mm film camera, or even a 35mm sized digital sensor without causing some major vignetting. 35mm SLR shooters who converted to DSLR's when they were first coming out were shooting with their 35mm SLR lenses and the pictures were coming out just fine in most cases. These photographers didn't realize their images would look like crap until the lens manufacturers started telling them that they needed as special digitally optimized lenses that are DSLR specific for their images to look right! So what gives? Were the lens makers right or was it just a bunch of marketing hype? What if I told you it was actually a little of both? What lens manufacturers said was right, but only some of the time for certain things, so mostly their claims were (and still are) fairly exaggerated.

Most 35mm film SLR lenses will work fine on your DSLR without any noticeable degradation in image quality. Sometimes the lens will actually outperform DSLR specific lenses! The reason for this is actually pretty simple. DSLR lenses are sized down to make the lens elements work with the same image circle crop as a 35mm SLR lens does on a 35mm film SLR. Unless you are using high-end pro level lenses, this can cause loss of sharpness near the edges of the image. Most all consumer level lenses suffer from this malady, with some being more noticeable than others. Light enters the lens from sharper angles, therefore not only can sharpness be reduced, but chromatic aberrations can occur. This where the lightwaves of different colors strike the film plane at slightly different positions than what they should. DSLR specific and digitally optimized lenses try to fight this problem with special coatings that reduce these aberrations as well as reduce the amount of reflections from light bouncing around in the lens. The chromatic aberration (color fringing) problems are most apparent when using wide-angle lenses, or shooting into bright lights, whereas edge softening is more apparent with telephoto lenses. So how can a 35mm SLR lens be better than a lens specially engineered for DSLR cameras only, to help overcome these problems? Simple, 35mm SLR lenses and especially those that are "digitally optimized" with special reflection reducing coatings only use the "sweet spot" of the lens. Remember, all lenses are sharpest closer to the center of the lens and image quality starts to fall off towards the edges. (Again, this is most noticeable with consumer grade lenses since the expensive pro-grade lenses are built to much tighter standards to reduce or even eliminate edge softening.) So when a lens built to throw it's image circle on a 35mm sized frame is used on a DSLR with a smaller APS sized sensor, only the light from the center of the lens reaches the digital camera's sensor, resulting in a sharper image from edge to edge.

So should you forget about those DSLR specific lenses and only used 35mm lenses? Absolutely not! Though you can get away with it most of the time, there will be times when a DSLR specific lens has it's advantages. First of all a DSLR lens is smaller, so there is less material involved reducing the cost of the lens to a certain degree. This also means that DSLR only lenses can weigh less than their 35mm counterparts, something to consider if using the lens for extensive travel or all day carry. Wide angle DSLR only lenses are usually considerably less costly than that of a 35mm wide angle lens. The DSLR only wide angle lens will most likely give better results, since it's special coatings and the careful alignment of the internal elements will do a better job of reducing chromatic aberrations (color-fringing) than the 35mm lens would. Again, a digitally optimized 35mm lens would be a fine choice too, if you don't mind the extra size and weight of the lens, this is especially true when considering telephoto lenses in the 200mm or longer range which can get quite bulky and cumbersome.

Basically, the choice is yours to make. Which lens meets your needs and budget? What is more important to you, image quality or portability? Are you keen-eyed enough to notice the differences? Do you really need the critical sharpness of a pro-lens for what you are photographing, or will a consumer-grade lens be more than sufficient for your needs? Unless you are shooting for absolute detail, the slight softness of a consumer lens will not matter nor be noticed. Aunt Hilda might actually prefer the softness imparted from a consumer lens used to take her informal portrait at the latest family gathering, because the fine lines and wrinkles on her face might not be recorded with the tack-sharp detail a high-end lens might capture. Most amateur photographers do not have the trained eye to notice minor differences in sharpness and clarity of detail, or pick up on minor aberrations, while professionals can sometimes be overly critical and demanding of the image quality. Notice that most DSLR specific lenses are consumer grade, while pro lenses are normally 35mm/DSLR interchangeable (digitally optimized) lenses. Also consider that the majority of brand name consumer-level lenses still have better optical properties than the professional lenses of 40 years ago. Again, the average casual viewer (or customer for that matter) will not be able to tell the difference between a photograph taken with a $150 consumer lens or a $1,000 professional lens. Only a trained eye knows what to look for in most instances.

When it all comes down to it, the camera is only as good as the lens that is put on it and the photographer whom is using it. The best camera in the world won't take decent pictures if the lens is of poor optical quality or the photographer lacks the proper skills necessary to take quality photographs that go beyond the average snapshot. A Nikon D70 with a consumer-grade Nikkor DX lens will take images of better quality than the average amateur photographer could ever hope for. On that same token, a Nikon D70 with a pro-grade Nikkor lens will take even better photographs. That same D70 with a pro-grade lens will produce sharper and better images than an expensive pro-level Nikon D2Xs with a cheap consumer-grade lens. Match the lens to the situation, and remember it's the camera's job to provide the specific exposure readings and to capture the image, while it's the lens' job to provide the image to the camera, and it's your job to make sure everything is composed properly and to skillfully use the equipment you have. And yes, skill has a lot to do with it. One last thing to consider: Most pros with that same consumer grade $700 Nikon D70 and cheap zoom lens can usually take better photos than your average amateur with a $4,500 Nikon D2Xs and expensive pro lens could ever dream of doing. This is why I laugh whenever I see the soccer-mom/receptionist/whatever amateur photographer using high-dollar equipment to take snapshots at the fair. It's like owning a Lamborghini for an everyday grocery-getter; it's both unnecessary, impractical, and serves no logical purpose. It's also more likely to get stolen (i.e. ripped off your neck) than the person 5 feet away with a cheap point-and-shoot, since all "soccer mom" is doing is trying to show off how much money she has to blow while she takes her crappy snapshots that will turn out looking just as bad as those taken by the person with the cheap point and shoot. In fact, the person with the cheap point and shoot camera might actually take better pictures because maybe he just happened to use the money he saved buying it, instead of a pro-level DSLR, to take some photography courses.

My advice: Work on your skills first, and don't worry about equipment details until you have the necessary abilities to take advantage of those differences.