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MarceloRSC Feb 21, 2010 5:26 PM

Cropping / Focal Lenght Multiplier in lens specifications
I've read about the Focal Lenght Multiplier, or "cropping", that is the reduction factor used to allow to use lens from 35mm film in digital sensors. It reflects the proportional lower area of the digital sensors (CCD or CMOS). Multiplying it by the focal distance we can obtain the 35mm equivalent.

In lens directed to film cameras, used in dSLRs it makes sence. So if I buy a 28mm destinated to 35mm, and use in a digital with 1.5x crop factor it will work as a 42mm lens due the image lost around the sensor.

But my doubt is:

Are the specs provided for new lenses directed to dSLRs affected by this factor? Or the zoom informed already was corrected to the kind of camera it targets?


At B&H site there is the Pentax K-X, that has a Crop factor of 1.5x. In the same shop there is the Sigma lens 28-300mm, directed for Pentax AF SLR. Is this zoom specifications (28mm and 300mm) already converted to the camera it is direcetd (a dSLR with APC sized sensor), OR these specs are really for 35mm film, and I need to apply the Focal Lenght Multiplier, obtaining an actual zoom of just 42mm to 450mm?

I'm confused, because there are lens that says clearly it's limits in real and 35mm eq. Example, the 18-55mm Pentax lens, that comes in some packs of the K-X, that informs to be 18-55mm and that is 27-80mm equivalent - in this case I can believe I'll have a 28mm similar to the ones I have in compact digitals (that informs the Eqivalent) or in my old 28mm film camera. But I'm not sure about the Sigma due this crop factor possibility...

Links from the example:

Lens well specified:
Sigma 28-300mm lens:

After all: if I buy this Sigma 28-300mm lens, is it what I'll get, or I'll have a 42-450mm equivalent?

Thanks and regards,


TCav Feb 21, 2010 5:40 PM

The focal length of a lens is a physical property of that lens, and does not change based on the size of the image sensor.

The "Crop Factor" or "Multiplier" are so that people that have a lot of experience with 35mm film SLRs can associate what they know about the lenses thay have with what they'll get with the same lenses on a dSLR with a smaller image sensor.

It serves no other purpose.

Putting the same lens on a camera with a smaller image sensor means you get a smaller angle of view. That's all. The focal length of the lens doesn't change.

shoturtle Feb 21, 2010 5:56 PM

You will get 42-450mm with the sigma on a 1.5 crop body. All lens range are give on the 35mm standard. And you will have to convert from the with 1.5, 1.6 with the 2 aps-c and 2x with 4/3 sensors.

rjseeney Feb 21, 2010 5:59 PM

Lenses are still referred to by actual focal length, even those that are intended for use only on DSLR's. So you still have to apply the "crop factor" to those lenses as well to get the 35 mm equivalent. As was mentioned, this was developed because everyone was more familiar with 35 mm film cameras.

TCav Feb 21, 2010 6:01 PM


Originally Posted by shoturtle (Post 1055326)
You will get 42-450mm with the sigma on a 1.5 crop body. All lens range are give on the 35mm standard. And you will have to convert from the with 1.5, 1.6 with the 2 aps-c and 2x with 4/3 sensors.

There is no conversion necessary. A 28-300mm lens will give the same angle of view on an APS-C dSLR as a 42-450mm lens will on a FF dSLR or a 35mm film SLR, but the lens doesn't change. If you have no experience with 35mm film cameras, then don't worry about "Crop Factor". It doesn't mean anything to you.

shoturtle Feb 21, 2010 6:11 PM

a 50mm prime on a 1.6 body is 80mm equivalent when I switch them between my eos 1 and T1i. So if you buy said a sigma 28-300mm and want to know what the aspect view will be on a 1.5 crop bodn they you need to do the quick conversion to get the range as it will not give you what you would expect form a 28mm on a ff body. No the lens does not change but the aspect does between crop and uncrop.

TCav Feb 21, 2010 6:44 PM

Your 50mm prime on your EOS 1 has an angle of view of 46. When you mount it on your T1i, it still has a focal length of 50 mm; it just has an angle of view of 29.

You need to do the quick conversion; MarceloRSC doesn't.

You have experience with 35mm film SLRs, and you move back and forth between the two worlds. MarceloRSC doesn't have a 35mm film SLR or a 'Full Frame' dSLR. He only has an APS-C dSLR, so he doesn't have to worry about 'Crop Factor' because he doesn't know what will happen with his lenses on a FF or 35mm camera, and he has no reason to even care.

shoturtle Feb 21, 2010 6:47 PM

Actually he does have slr film experience form his other thread. So I think he may notice the difference.

mtngal Feb 21, 2010 6:48 PM

As both TCav and rjseeney pointed out, what you get is a change in angle of view. While the 50 mm lens gives you the same field of view that you would get with a 75 mm lens on a film SLR camera, it won't give you the same depth of field at equal apertures (the 50 mm lens would give you a deeper dof at f8 than a 75 mm lens would on a film camera at f8, at least according to my dof calculator). I wouldn't worry about the crop factor when choosing lenses either. I'd be far more concerned with the quality of the individual lens.

MarceloRSC Feb 21, 2010 8:10 PM

Thanks for the replies!

The doubt is actually about the notation, not the concept of the crop, that I think I understood. The "35 equivalent" usually shown besides any lens specification gives to the experienced photographer a REFERENCE that may be easily compared about the field of view - the actual angle of view. Despite an angle would be a more direct and constant unit to measure it, it is not so popular as the zoom in milimeters, that unfortunately depends to the size of the capture area.

I actually have a reference about WHAT MEANS 28mm in a 35mm camera, in practice, since my older film SLRs or P&S film cameras - I always prefered 28mm cameras, even the compact, because they are more useful and flexible for casual photos. That's why I'm looking for anything that starts at 28mm (eq!) now for a dSLR.

When migrating to digital, I also looked for 28mm EQUIVALENT, and always got simmilar fields of view, like the Olympus C5060WZ or the compact Sony W170, ALL of them 28mm EQUIVALENT. And since then I learnt about the concept of "equivalent", due the proportion between the sensor and the film. But in all specs of these fixed lens cameras both measures are always shown in the real mm and the equivalent mm, making this task easy. The same occurs with the newest WideZoom, like Nikon P90 (starts in 26mm EQ).

The new fact now, when I'm planning to return to the world of SLR is the difference in the sensor sizes of the dSLRs, and the fact that some lenses can be used in cameras with different sensor sizes. As I pointed, some lenses (as the original Pentax) gives clearly the specs, both the real and the 35mm equivalent to the dSLR it is intended, so I have no doubt that if I get a 18-55 I'll see the same I've been seeing in my 28mm all these years.

And the doubt came from the Sigma mentioned (and happens with others too), when ONLY one spec is shown, with no reference to the word "equivalent". So I didn't know the exact meaning of this, and what I should expect.

Based on your responses (thanks all!), it seems that the specified mm on these lenses are NOT "EQUIVALENT" - they are the actual specs for a 35mm film. It is not "equivalent" because it is not converted to any camera. So I need to apply the crop factor to calculate the "equivalent" zoom for the model of SLR I am wondering, based in it's particular sensor size. So the 28-300mm in this Sigma lens is in the same "unit" than the 18-55 in the original, and must be converted. I thought that MAYBE it could be already converted as it is directed to a dSLR brand, but it makes more sense that it is not converted.

It is really BAD news, because a 28-300mm would be a very flexible lens, while despite the powerful tele, a 42-450 is not what I'm looking for...

As I posted in the other topic (dSLR x Prosumer) I was considering the SLR with such lens to play instead a Wide Zoom prosumer, because I thought the size of this 28-300mm rasonable, but now I'll need to look for a 18-200 instead to have the results I was wondering...

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