# Digital vs Film Lenses



## carusoswi (Sep 9, 2007)

Ok, I understand the crop factor as it applies to the use of lenses developed for 35 mm film cameras on digital SLRs.  What I don't know about is what changes are designed into a lens that is developed for DSLR's from the beginning.

Many a salesman has stated that the long end of my 28-300 hyperzoom film lense equates to 450mm on a DSLR.  Obviously, the lense hasn't changed, so the effect is purely due to crop factor caused by the smaller image pick-up area of the DSLR sensor.

So, if a lens is "optimized" for a DSLR, are the same variables in effect with merely the opposite result?  Would not a DSLR 450 mm lense have a magnification factor equal to that in the optics of a 300 mm SLR lense?

It seems obvious to me that the image area covered by a DSLR lens is not going to fill the image area of a 35mm slr cam, but, if the magnification power of a 300 mm SLR lens remains constant, no matter the type of camera upon which it is mounted, would not the reverse be true . . . that the magnification factor of a 450 mm DSLR lens would be no more (or less) when that lens is mounted (if it can be physically mounted) on a 35mm film body camera?

So, in other words, if you were to crop out the central part of an image captured in both 35mm and DSLR formats, would not the actual size of the image remain constant between the two formats?

My question is purely theoretical, I understand.

But, it brings to mind other questions . . . such as, why were DSLR's developed not to conform with the aspect ratios of the well-established 35mm format that they have ultimately supplanted?

Thanks for any insights.

Caruso


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## jstuedle (Sep 9, 2007)

To answer your last question, smaller sensors are easier and cheaper to manufacture. And yes a 300 mm lens designed for film or digital is still 300mm and the relative focal length is 300mm in full frame and 450mm on a 1.5X factor APS size sensor. I have never bought into the DX format lens, and will most likely not ever. I want to use any lens I own on both film, cropped chip, and full frame sensors. The only changes for DX type digital lenses is smaller image circle, lighter weight and smaller size because of the smaller image circle, and in some cases improved coating. The coating enhancement will help film as well as digital, but it's marketed as improved coating for digital.


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## Alex_B (Sep 9, 2007)

carusoswi said:


> So, if a lens is "optimized" for a DSLR, are the same variables in effect with merely the opposite result?  Would not a DSLR 450 mm lense have a magnification factor equal to that in the optics of a 300 mm SLR lense?




As you said, the sensor area is smaller than the area covered by 35mm film, that means, a lens which is made for the smaller sensor only, usually has a smaller image circle. that means that it will not project enough image to fill the whole area of 35mm film.


a 300mm lens on 35mm format gives you a wider angle of view than a 300mm on a smaller sensor... this is purely due to the difference in medium size. the focal length and hence the magnification in the image plane remain exactly the same.

however, when you then print (or view on screen) the image, then the image from the larger medium (35mm film) has to be magnified less than the image from the smaller medium (small sensor). hence only in this step you get an increase in magnification which coresponds to something like going from a 300mm lens to a 400something mm lens.


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## Alex_B (Sep 9, 2007)

carusoswi said:


> But, it brings to mind other questions . . . such as, why were DSLR's developed not to conform with the aspect ratios of the well-established 35mm format that they have ultimately supplanted?



because, as with all high-tech semiconductor devices, larger sensor area costs unproportionally more money in the production process.


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## Alex_B (Sep 9, 2007)

carusoswi said:


> So, if a lens is "optimized" for a DSLR, are the same variables in effect with merely the opposite result?  Would not a DSLR 450 mm lense have a magnification factor equal to that in the optics of a 300 mm SLR lense?



no, because a 300mm lens is a 300mm lens. focal lengt remains focal length and does not depend on the image circle the lens projects.


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## Garbz (Sep 9, 2007)

The difference between optimised for digital lenses such as the Nikon DX series comes from it's reduced image circle. Mount one of these lenses on a 35mm camera and you will see a black circle around the outside. The "optimised" bit comes in that this reduced image circle can now be made in a much smaller form factor than the equivalent 35mm lens. Smaller and lighter with same quality = more sales to consumers.

However I suggest that you do not strongly buy into these lenses unless you have an Olympus camera, since the company is strongly pushing the 4:3 system. Canon and Nikon both now have digital cameras in their lineup with 35mm sized sensors which would render the lenses unusable without cropping.

Also to put into perspective the cost. CCDs or CMOS sensors roll off a production line in a wafer. The wafer costs the same to produce and will likely have an array of chips, maybe 50+ APS sensors on it, or 200-300+ P&S sized sensors. On top of that there will be electrical faults so the yield on the sensors may only be 80-90% or so. Suppose now you increase the sensor size. You can now only get 30 sensors on the wafer but out of these 30 sensors the yield goes down too because of their size the error rate increases. So rather than having 40+ good sensors come off a wafer you only get 15-20. But the cost remains the same. Despite what some people think the excessive price of the D3, 5D or 1Ds is not simply because someone wants to make more money.

Same thing with megapixel count. The process is similar but when you start making things smaller the error rate goes up and the yield drops.


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## EOS_JD (Sep 9, 2007)

jstuedle said:


> a 300 mm lens designed for film or digital is still 300mm and *the relative focal length* is 300mm in full frame and 450mm on a 1.5X factor APS size sensor.


 
The focal length does not change. All that changes is the field of view.....  Smaller sensors have a smaller field of view.


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## DonSchap (Sep 9, 2007)

Maybe this image will help explain:







The lens originally developed for 35mm-film cameras offers the image circle. A Full-Frame digital sensor makes use of the full image circle, while an APS-C sensor only take about 60% of the center (called a crop). The image rendered from this crop appears larger ... but really is not ... it just took less of the image (an apparent increase in _focal length_ known as the "digital cropping factor").

If the image circle was provided by a 300mm lens ... the resulting APS-C "crop" image appears to have been taken by a 450mm. You lose all the image around the "crop" ... because there is no sensor there to get it.

Lenses "optimized" for APS-C sensor even lose the full frame image circle, because they push it through a smaller lens opening at the rear of the lens. Less glass (because you don't need all that on an APS-C sensor body) = less cost. An when you are built a million lenses ... that can add up quick. 

If you place an APS-C optimized lens on a full frame or 35mm-film camera body, the image will vignette (blackened corners), similar to this:






I hope this helps.


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## fmw (Sep 10, 2007)

There is no difference between a digital lens and a film lens other than the angle of coverage, sometimes known as the image circle size.  There is nothing about digital photography that changes lens requirements.  Digital changes the recording medium, not the photography.

Others have described how the "crop factor" works so I won't add anything to that.


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