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12-20-2009, 05:05 AM   #1
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Asked once but I must ask again :(

Ok I thought I had it all figured out what lenses did what and how to calculate "crop factor" on 35mm-APS-C lenses.

However, I am horribly confused as to what lenses are actually "made" for digital cameras. So for example if I want a
28mm, what "digital" lens do I have to buy to get an actual 28mm??

Basically, what Pentax and third party lenses do I need to look for now that I have a digital?

This digital thing is really becoming a confusing learning experience.


12-20-2009, 05:25 AM   #2
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A 28 mm lens is always a 28 mm lens. That value represents its focal length, which is a property of the lens, not the camera.

What changes with the so-called crop factor is the FIELD OF VIEW.

In other words, a lens designed for film creates an image larger than what the DSLR's sensor can capture. the DSLR is effectively cropping a part of your image, but the lens doesn't know that. A lens designed for digital simply creates an image size better matched to the sensor's size. The lens itself is cropping, if you will.

For the record, lenses create circular images so even film crops part of it.

To summarize, the properties of a lens are fixed and do not depend on the camera placed behind it.
12-20-2009, 05:31 AM   #3
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A 28mm is a 28mm is a 28mm.
12-20-2009, 05:35 AM   #4
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Here is a quick cheat sheet for your dSLR:

16mm- = ultra wide
18mm = wide angle
24mm = moderate wide angle
35mm = normal
90mm = moderate tele
150mm+ = long tele

The zooms follow the above numbers.

This is the case regardless of whether a lens has a "D" in the name. For example, the Pentax-FA 35/2 (originally designed for 35mm film) and Pentax-DA 35/2.8 (designed for APS-C) both have the same field of view (FOV) on your camera. This is also true for non-Pentax lenses (e.g. Sigma DG vs Sigma DC).


Last edited by stevebrot; 12-20-2009 at 05:44 AM.
12-20-2009, 05:51 AM   #5
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To get the equivalent field of view of a 28 mm lens on a 35 mm camera, you need an 18 mm lens on a Pentax DSLR.
12-20-2009, 05:59 AM   #6
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QuoteOriginally posted by Raptorman Quote
So for example if I want a
28mm, what "digital" lens do I have to buy to get an actual 28mm??

To simply give you a direct answer, 18mm (digital value) = 27mm (full frame 35mm film value). (Although 18mm is 18mm and 28mm is 28mm.)

Since I see in your public profile that you were (or are) a film shooter as well, you need 1 simple comparison calculation to straighten it out for you. A ___ mm lens on my 35mm full frame, film camera divided by 1.5 will give me an equivalent view on my digital camera. For most people its easier the other way around. if I buy a 20 mm lens (for my DSLR) it will appear like a (20 X 1.5 =) 30mm in my minds eye (trained under a film SLR).

The next step is to figure out why this is true. So while your out there shooting with your 18 mm lens, start reading (learning) how a lens works and how a camera uses a lens (assuming you really want to learn photography, not just how to take pictures).

Good Luck & happy shooting,
12-20-2009, 11:24 AM   #7
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Unless you also shoot a film camera or correspond with people who do and wish to have a common basis for talking, forget you ever heard about a crop factor.

The answer to your question about 28mm depends entirely on what makes you think you want a 28mm lens. If you mean, "I know what 28mm looks like on film, and I want something that provides the same on digital", then you do in fact need the crop factor, and it's 1.5. Meaning an 18mm lens is what you want (which is why that's where the kit lens starts).

Whether the lens was made for digital or not has *nothing* to do with any of this. All 28mm lenses provide the same field of view on digital, whether the lens was made for digital or not. Similarly, all 18mm lenses provide the same field of view on digital, whether the lens was made for digital or not.

What you need to let go of is the notion that there is such a thng as an "actual" 28mm. as has been said, 28mm is 28mm. They are *all* actual 28's. It's just that a 28mm lens will have a different field of view between film and digital. So if what you really mean is, "the same field of view a 28mm lens provided on film", then that's how to say it, and that's when you need the crop factor.

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