effective focal length question

I have searched a number of threads and learned quite a bit about digital, but I remain confused about "effective focal length' of non-digital lenses on digital bodies. I realize the sensor on the D70 is smaller than 35mm film, but how does the effective focal length come into play? I would think that only the field of coverage (or angle of view) for that lens would change, not the magnification.

If I put a 100mm lens on a digital body, I have heard that this would change the effective focal length to 150mm (1.5x). Is this correct? Does my 80-200 with 2x TC now give me the reach of 240-600 mm? Or do I merely get the coverage of that range? If additional magnification occurs...how does it occur?

Thanks for helping a newbie?

JBlake

Your comment is correct, except that I believe the actual change in effective focal length is about 1.4x: "If I put a 100mm lens on a digital body, I have heard that this would change the effective focal length to 150mm (1.5x)."

In a way it's not unlike switchig to different film formats. A 100mm 35mm lens does not have the same field of view as a 100mm lens in 645 format. Since you can't use a 35mm lens on a 645 camera is really doesn't matter though.

This is a mixed blessing for those that are switching from film to digital. It's really great for people who like to use longer focal length lenses. But you still end up having to switch around your lens inventory a bit in order to get the same coverage for digital as you had with film.

Thanks for your reply, Jeff82, but I am still unclear. Lemme ask it another way...if I put my 300mm on a digital body, does that bring me 1.4x closer to the action?

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Originally Posted by JBlake

Thanks for your reply, Jeff82, but I am still unclear. Lemme ask it another way...if I put my 300mm on a digital body, does that bring me 1.4x closer to the action?

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No. It will still be 300mm but with the edges cropped off.

Thanks Michael. That's what I suspected.

I've been calling it the "digital crop factor". I think that's more accurate. It varies with the camera, too. I believe the Nikons all have a crop factor of 1.5x, the prosumer Canon's have a crop factor of 1.6x, the Canon EOS 1D and 1D Mark II are 1.3-1.4x, and I'm not sure about the Pentax *ist D.

John, have you heard any information/feedback about the new "digital" lenses? Sigma recently released 2 lenses that are supposed to correct for the digital sensor size.

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Originally Posted by kkraczek

John, have you heard any information/feedback about the new "digital" lenses? Sigma recently released 2 lenses that are supposed to correct for the digital sensor size.

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Sure. Canon's and Nikon also have lenses designed specifically for digital SLRs. The focal lengths are still expressed in 35mm terms (I guess that's how you say it), but the "image circle is smaller. That means the digital-specific lenses won't provide full coverage on a 35mm film camera. But they will be wider than what's possible with 35mm lenses.

Hope that made sense.

Yep, it did. :) I guess I'm hoping someone here will have used them and can give an idea of how well they work. I may just buy them anyway and be the first to review! Thanks!

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Originally Posted by kkraczek

Yep, it did. :) I guess I'm hoping someone here will have used them and can give an idea of how well they work. I may just buy them anyway and be the first to review! Thanks!

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Krsitin,

Mig has the Sigma 12-24, and she posted some examples of it. The thread is here.

Wow, thanks Sebastian! That's what I was hoping for... :)

Hi Kristin,

FYI - The main difference between the 12-24 DG and the ones you're looking at is that the 12-24 works with film bodies. Since I also have an EOS 3, it was important to me that it be compatible. Like you I was a little worried when I ordered it because while there were some reviews here they weren't by anybody I knew. But I googled it and combining PR's reviews with what I saw at other places convinced me to go for it - I just love wide. :) Good luck!

Danielle

Please let me know if I've got this straight... if you were to print out a digital photo at the same size as a 35mm format, it would appear that you were closer to the action. So it's only a bad thing if you're going for wide angle shots. Otherwise, I suppose that one could just physically back up in order to get more of the subject in the frame?

Also, does the viewfinder accurately reflect what will be in the final photo?

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Originally Posted by kkraczek

Please let me know if I've got this straight... if you were to print out a digital photo at the same size as a 35mm format, it would appear that you were closer to the action. So it's only a bad thing if you're going for wide angle shots. Otherwise, I suppose that one could just physically back up in order to get more of the subject in the frame?

Also, does the viewfinder accurately reflect what will be in the final photo?

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1) Say you shot with a 200mm lens. On a DSLR, it would look more like you shot it with a 200mm and cropped it to the angle of coverage of a 300mm, rather than shooting it with a 300mm. The difference is in the perspective. It's probably more apparent with wide lenses but not exclusive to them.

2) You could back up to get everything in the frame - if that's an option (maybe not inside a building or on the edge of a cliff, etc). Once again, there's those perspective differences...

3) Viewfinders in all SLR's (film or digital) probably don't reflect what the shot will be for a lot of reasons. First off, unless it's a top of the line camera, you're not seeing 100% of what's going to be in the image. Maybe 89% like my S2 or 96% like my F100 (or close) but not 100%. Second, the exposure matters of course. Third, color balance - and the list goes on. But - the advantage of an SLR is that you are looking thru the same lens that's taking the picture so - other than what I listed - you will get what you see. (Did I answer your question or confuse you!?)

LOL- no, I think I've got it- thanks! :)

[QUOTE=another view]1) Say you shot with a 200mm lens. On a DSLR, it would look more like you shot it with a 200mm and cropped it to the angle of coverage of a 300mm, rather than shooting it with a 300mm. The difference is in the perspective. It's probably more apparent with wide lenses but not exclusive to them.

If you shoot from the same distance, 200mm on a digital is exactly the same as 300mm on film (assuming a 1.5 cropping factor) because they have the same angle of view.


Simon

[QUOTE=straightarmIf you shoot from the same distance, 200mm on a digital is exactly the same as 300mm on film (assuming a 1.5 cropping factor) because they have the same angle of view.[/QUOTE]

You're partially right, the angle of view is the same. And with a 200 vs a 300, the perspective will be very close. Between those two lengths it's not something that you'd really worry about in the real world. The different perspective is more obvious with really wide lenses.

But, shoot a subject with a 200mm lens on a DSLR with a 1.5 crop factor, not filling the frame - leaving some space in the background around it. Now shoot the same scene with a 300mm lens on a full frame DSLR or 35mm SLR and keep the subject the same size in the frame as with the 200mm. The background will be different due to the difference in perspective of those lenses. There would be a more noticable difference between a 20mm and a 28mm than this example though. Or 200mm vs. 400mm, etc.

not of focal length or effective focal length.

If you shoot two images from the same distance the perspective is the same whether you use a 20mm, 200m or 800mm lens.

The only way you can change perspective is by moving closer or further way.

Simon