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03-31-2007, 06:43 AM   #1
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I'm confused, DSLR focal lengths clarification

Last night I was at a concert, and shooting a few with my kit lens, and really missing any kind of telephoto length. I'll post some images later, when I can find some I like.

Anyway, there were 3 other camera guys running around, and all had either 200mm or 300mm lenses (I've got the only Pentax in the organization). One guy was extolling the virtues of a 2.8 lens and low light... and then he started talking about the difference in focal length between digitals and film cameras, and promptly confused me.

So, for clarification... Due to the size of the sensor, when placing a lens on a DSLR body, if the lens was made for a 35mm application, the rule of thumb is to multiply listed focal length by 1.5? That is an 80mm lens actually gives 120mm focal length.

Or are you supposed to divide by 1.5? I was all confused when he got done.

Also, if the lens is made for a DSLR, won't the focal length be accurate?

03-31-2007, 07:00 AM   #2
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Hi, here is a link to a thread where you can find an answer to your puzzle.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/5065-wanting-under...op-factor.html
03-31-2007, 07:13 AM   #3
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It's very confusing, and there is at least one other thread on these forums besides the one Kurt mentioned if you do a search.

All lenses, when placed on a digital body, are subject to focal length multiplication. With the exception of the C*non 5D (which is full frame) this is because the size of the sensor inside the camera is different from the size of a 35mm frame.

What this amounts to is that the field of view on a digital slr will appear to be that of a 35mm with a focal length 1.5 times the length you are using. For example, an image shot at 200 mm on digital will look like an image shot at 300mm (1.5 times 200) on film. This is the reason why the kit lenses packaged with digital SLRs tend to have 18mm focal lengths at the short end instead of 28 with was common with film. Its also the reason people hunt for 10, 12, 14, 20mm lenses if they want a very 'wide' lens, because when mounted on a digital, these lenses appear to have the same fields of view older 20-24mm lenses had on film. However, try not to get hung up on focal length too much. Focal length is focal length is focal length. The field of view is what changes.

Also, if a lens is made for digital-only, aside from the adjustments in focal lengths I just mentioned (18 vs. 28 kits, 14 vs. 21), the lens overwhelmingly won't have an aperture ring, and there is one other difference. Imagine a square (rectangle) circumscribed within a circle. The rectangle is your frame of film, or sensor inside your digital camera. The circle is the circle of light that can be thrown upon that frame. The digital 'square' is smaller than the film 'square', and the circle that encompasses the film square is big enough to cover both the digital and film squares, but not vice versa. Digital only lenses only have a 'circle' big enough to cover the digital frame, so theoretically they can be made smaller and lighter, and are unsuitable for film frames because they can't throw enough light to cover them completely.

Last edited by bdavis; 03-31-2007 at 09:40 AM. Reason: slight wording change
03-31-2007, 07:42 AM   #4
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Very nice explaination Beth. Clear and consise.

03-31-2007, 08:02 AM   #5
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QuoteOriginally posted by bdavis Quote
Also, if a lens is made for digital, aside from the adjustments in focal lengths I just mentioned (18 vs. 28 kits, 14 vs. 21), the lens won't have an aperture ring, and there is one other difference. Imagine a square (rectangle) circumscribed within a circle. The rectangle is your frame of film, or sensor inside your digital camera. The circle is the circle of light that can be thrown upon that frame. The digital 'square' is smaller than the film 'square', and the circle that encompasses the film square is big enough to cover both the digital and film squares, but not vice versa. Digital only lenses only have a 'circle' big enough to cover the digital frame, so theoretically they can be made smaller and lighter, and are unsuitable for film frames because they can't throw enough light to cover them completely.
This isn't always true. The D-FA Macro lenses are 'made for digital' but have aperture rings and can be used on 35mm film cameras. The DA40/2.8 can be used on 35mm film.
Many Sigma and Tamron 'made for digital' lenses likewise have aperture rings and can be used on film without vignetting.

It is a disturbing trend in general that lenses are losing aperture rings. I really think that soon no lenses will have aperture rings, mainly because even the lowest camera models have automation to control it. This will come to pass for 'full-frame' and cropped sensors equally.


To the OP - focal length is focal length, period. If a lens is listed as 50mm, it doesn't matter if it is made for medium format, 35mm film, or digital - it will give you the same view on your camera.
03-31-2007, 09:50 AM   #6
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Thanks, Alan. I hope my info was helpful to the OP.

carpents, as with most things, there are always exceptions. Yes, there are several lenses that bridge the gap between film and digital only in various ways (coating and lens construction to avoid flare/ghosting are the biggies). The absence of aperture rings is really more a trend due to cameras with automation than due to digital only lenses, though, as you note.

I have amended my first post to refer to digital-only lenses, which is a bit more accurate. I've always found the issue of the DA40 to be more of an anomaly than the rule. Yes, it does work, but it isn't billed as a D-FA.

Last edited by bdavis; 03-31-2007 at 09:59 AM.
03-31-2007, 10:09 AM   #7
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Thanks for the help.

So, the reason my kit lens is 18-55 while most film kits are 28-80 is the sensor and related image size. Totally get the circle of light explaation, very logical, reminds me of a syllogism (or whatever it's called).

If I understand it, a 28-80mm film lens will give a slightly increased zoom capability? Since I could buy one cheaply... is it even worth doing, or should I just live with short zooms until I can get a true telephoto?
03-31-2007, 10:35 AM   #8
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Well, you'd have 25mm more reach. It isn't that big of a jump, but it depends on how much more reach you need. You said your buddies were shooting with much longer lenses.

What do you want with the tele? Do you also need a faster lens? You can find good f/2.8 28-80mm lenses, but not as cheaply as those 28-80mm f/3.5-6ish.

03-31-2007, 11:14 AM   #9
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I have little money, but like at the concert, I want to be able to be unobtrusive, but still get great shots. So, a faster lens would be better.

On a side not, I just came back from the local flea market where I had heard a guy was selling a lot of camera gear. He was, and even had some Pentax lenses. He had three older K series manual focus 200mms for about $60. He's there every saturday, so I might be going back with my camera and trying one of those out. Realistically, a 200mm is probably going to get me closer to the range I want to be anyway. I know they won't be as good as some out there, but... for $60?
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