Crop Factor on Designed for Digital Lens ???

[trublubiker]

Being very new to DSLR photography I'm still trying to get my head around all the technicalities.

I understand that lens built for 35mm capture have a 'crop factor' when used with an APS-C type sensor, due to the reduced angle of view, ie your 100mm lens becomes 150mm with a 1.5 crop factor.

Does this factor still apply to lens that are specifically designed for use with digital sensors?

I am aware that Canon and Nikon have '35mm size' sensors on their 'Pro' bodies. Are the digitally designed lens being built for these bodies, and if so, does this indicate that the industry is moving toward all DSLR's having 35mm size sensors ?

I note in the Tamron catalogue that their Di11 lens are "designed for exclusive use on digital SLR cameras with APS-C size imagers". Would these lens then have the effect of increasing the angle of view on the bodies using full frame sensors? Would your 100mm lens become a 66mm lens?

Hope I haven't got everyone else as confused as I am !

trublubiker

[wheels]

As understand it, if you fit a lens designed for an APS-C sensor to a full frame camera you'll get vignetting at lower end. For example if you fit a Canon EF-S 10-22mm to a 20D/400D/450D it will work just fine. If you fit that same lens to a 5D/1D you'll get vignetting between 10-15mm.
As I said, thats as I understand it. Happy to be corrected. If you currently own a APS-C camera but may be upgrading to full frame some time in the future you may wish to steer clear of those type of lenses as they will probably have to go with the camera when you upgrade.

[ATJ]

If I understand the question correctly, yes, the focal length for a DX type lens is still relative to 35mm film (36x24mm frame).

So a 100mm DX lens will give the same angle of view on a DX camera as a normal 100mm lens would on the same camera.

[Shoot]

I note in the Tamron catalogue that their Di11 lens are "designed for exclusive use on digital SLR cameras with APS-C size imagers". Would these lens then have the effect of increasing the angle of view on the bodies using full frame sensors? Would your 100mm lens become a 66mm lens?

These lenses can't be used on a Full Frame Camera because the mirror usually hits the back of the lens they, can only be used on crop body's.
If you have a 50mm Full Frame Lens and use it on a 1.6 Crop Body the field of view you get is equivalent to an 80mm Lens. So, using Full Frame lenses on a Crop Body you'll find your wide angle suffers dramatically.

If you were able to ( and there are some hacks out there to do it ) use a APS-C (only for Crop Body) Lens on a FF Camera the image circle projected on the 35mm sensor would not cover it completely and would cause severe vignetting so there's no real advantage in doing that, you'd need to crop back to get any usable image.

[Glen]

Trublu, there are two things here and it is a matter of terminology. A DX (in Nikon speak) lens is designed for a crop sensor and provides a smaller image circle, as stated above. Many companies have updated their lenses to "designed for digital", some of those lenses suit both FX and DX, the main effect of designing for digital is greater use of anti reflective coatings as digital sensors both are more sensitive to and create more extraneous reflections. A lot of it is just marketing speak so look at the specs.

[Yi-P]

The markings on the lens(es) are based on its focal length optical formula. It has little to do with the 'format' of the image circle it produce on the back end.

Image circle is the image area the lens is designed to 'project' an image onto that specific plane, ie. film plane. A larger film plane takes up much more of the projection area as smaller ones. A point and shoot with 6mm lens would look similar to a 50mm lens on 35mm or 80mm on a medium format.
You can imagine a sheet of A4 paper is actual image projection plane of a lens, a crop factor is how big you draw the box inside that paper.

Same principle applies to full frame 35mm lenses and DX-Crop lenses. You can't print a full sized A4 image onto an A5 page, you will leave some areas that are cropped out (Using DX lenses on full frame bodies). Though you can always crop an A5 size out of your A4 page (Using full frame lenses on DX bodies).

Though there are always physical and practical limits where you can push optics to.

[Mr Darcy]

The focal length of a lens has everything to do with its optics, and nothing to do with the camera it goes on.

For simple lenses, the focal length of a lens is the distance from the lens axis to the image plane when the object represented by the image is at infinity. Think of burning ants (or paper if you are a Buddhist child ) with a magnifying glass. The sun is effectively at infinity. When the image of the sun is focussed on the ant, the image is at its smallest, and most intense. The distance form the lens to the ant is the focal length of the lens.

Thus a 100mm lens will be focussed on infinity when the lens is 100mm from the film plane
Compound lenses muddy concept this a little (otherwise my 10mm lens would need to be entirely inside the body of my camera!) but the principle holds.

Because, until recently 35mm film cameras were ubiquitous, they are held to be the "norm"
Our 100mm lens covers an area within the angles of 25 degrees across the film (run two straight lines out from your feet to two objects on the opposite edges of the field of view. The lines will be 25 degrees apart.)

Now take that photo and trim 1/4 from each edge. The remnant photo will be half the size of the original and will be the SAME image as if you took the photo on a half 35mm frame camera, or a DX sensor camera using that 100mm lens.

IT will cover much the same field of view as if you used a 150mm lens on a 35mm camera. This is why the sales people talk about 150mm equivalent. However, the image will NOT be the same as the one taken with the 100mm on a DX camera. The difference will lie in the relationship between near objects and far ones. To see this, take a photo with a wide angle lens. Have an object in the foreground and one in the background. Now take the scene with a "normal" lens. Adjust the near object's position so it fills the same amount of the frame as in the first photo. Repeat with a telephoto lens. Now take all three photos and compare the relative sizes of the near object to the distant one. You will find that the WA makes the distant object appear small, while the TP makes the distant object seem big relative to the near object. Now take the "normal" shot and trim it so that it shows the same amount of background as the TP shot. The result will not be the same as what the TP does., BUT IT IS WHAT A DX SENSOR DOES!

Now back to the camera.
A DX sensor is about half the size of a 35mm (so called full frame) sensor. That means that all the effort to produce an image that will cover an FX sensor entirely is wasted if all you are using is a DX sensor. So that means that you can make lenses more cheaply and easily for the smaller sensor than for the big one. It also means that you might be able to come up with more outrageous solutions if you only need to consider a small sensor. (think 18-200 zoom) It also means that you can move the back of the lens closer to the film plane as the swing of the covering mirror is smaller. This might also allow otherwise impossible solutions. In short, you can use a lens designed for 35mm on a DX sensor, but not the other way round. The size of the sensor is such that it is similar, BUT NOT THE SAME, as using a 50% longer lens on 35mm. PS Yes I know that there are few, if any simple lenses on cameras these days, though I do have an ancient 500mm telephoto that has a single lens element in it. And yes it is 500mm long.

[ATJ]

IT will cover much the same field of view as if you used a 150mm lens on a 35mm camera. This is why the sales people talk about 150mm equivalent. However, the image will NOT be the same as the one taken with the 100mm on a DX camera. The difference will lie in the relationship between near objects and far ones. To see this, take a photo with a wide angle lens. Have an object in the foreground and one in the background. Now take the scene with a "normal" lens. Adjust the near object's position so it fills the same amount of the frame as in the first photo. Repeat with a telephoto lens. Now take all three photos and compare the relative sizes of the near object to the distant one. You will find that the WA makes the distant object appear small, while the TP makes the distant object seem big relative to the near object. Now take the "normal" shot and trim it so that it shows the same amount of background as the TP shot. The result will not be the same as what the TP does., BUT IT IS WHAT A DX SENSOR DOES!

Greg,

Sorry, mate. I disagree. Yes, there is a difference between how a wide angle lens sees a scene versus a telephoto, but when comparing shots with the camera in the identical position (as you would for a DX to FX comparison) that effect is negated. i.e. you have not moved the camera at all. This means that if you draw straight lines from the centre of the lens to any object in front of the lens, the lines will pass through the exact same points no matter what lens you have on the camera*. If you mount a camera on a tripod and shoot a scene (at a reasonable distance from the camera) with a wide angle lens, then a telephoto lens and then compare the shot from the telephoto lens to the one from the wide angle lens considering only the area of the latter that was captured by the telephoto lens and they will be identical (other than potentially depth of field depending on the apertures used). They have to be because light travels in a straight line between the scene and the lens.

In the same way, if you compare a 100mm lens on a DX camera and then replace the camera on the tripod with an FX camera with a 150mm lens, the image will be the same (expect for DOF).



* note that I'm talking about reasonable distances here (>1 metre). Obviously at very close distances characteristics of the compound lenses will cause some variation.

[digitor]

You've got some very complicated answers to your simple questions, but if I may attempt to answer a couple of your specific points:

I understand that lens built for 35mm capture have a 'crop factor' when used with an APS-C type sensor, due to the reduced angle of view, ie your 100mm lens becomes 150mm with a 1.5 crop factor.

Does this factor still apply to lens that are specifically designed for use with digital sensors?

Yes.

I note in the Tamron catalogue that their Di11 lens are "designed for exclusive use on digital SLR cameras with APS-C size imagers". Would these lens then have the effect of increasing the angle of view on the bodies using full frame sensors? Would your 100mm lens become a 66mm lens?

No.

And some closing remarks:

Focal length is focal length, regardless of the format which the lens is used on.

Perspective has nothing to do with focal length, the only way to change it is to change the distance between camera and subject.

Cheers

[trublubiker]

Thank you all very much for taking the time to reply.

I think I've gleaned enough from all the info provided to answer my questions.

I will admit though that I did get the feeling, momentarily, that I may have opened Pandora's DSLR Lens box.

I should have added that I've recently bought a K20D with the 18-55 kit lens and was not at all impressed with the build quality or the IQ, lens only, I'm super impressed with the camera ! I've also acquired 2 older manual focus lens, nice solid build and much better IQ. I came into DSLR from a Canon 'A' series background (still have about and am still getting used to the plasticity (is that a word?) of the modern gear. I suppose I'm re-adjusting my learning curve from 35mm to digital.

Thanks again for the help

trublubiker PS: And Mr Darcy, you win ! At 500mm, yours is bigger than mine.

[Mr Darcy]

G'day ATJ
Perhaps I didn't explain too well
I will use a book illustration (Mods please remove if you feel it breaks the rules, but I feel this falls under the fair dealing for study rule.)
The book:
Telephoto and Wide-Angle Photography; Robert Simmons; Chiltern Publishers; 1959; ISBN did not exist back then. The illustration on p93 Shows the effect on perspective of changing the focal length of the lens:
EDIT Scan removed. Educational purpose met. Refer Link in Digitor's post below for a better (and definitely legal use) images
To bring it into the digital age, this is (roughly) what a 50mm on a DX sensor would look like. Note that it is the centre of the 50mm photo above. Note that while it covers the same angle of view as the 85mm, the effect on perspective is quite different.
EDIT Scan removed. If you want to look at the book, I have a copy, or possibly your local library
It also has details of DIY telephoto lenses. Ahh. Those were the days

And Mr Darcy, you win ! At 500mm, yours is bigger than mine

Don't sweat it. (1). it is an old screw mount. I can't use it on anything I own these days. Digital or Film. (2) it is f11 (3) It is very poor image quality.

[digitor]

G'day ATJ
The illustration on p93 Shows the effect on perspective of changing the focal length of the lens:

This illustration in fact shows the effect on perspective of changing the camera position.

Cheers

[ATJ]

G'day ATJ
The illustration on p93 Shows the effect on perspective of changing the focal length of the lens:

This illustration in fact shows the effect on perspective of changing the camera position.

Cheers

Exactly, which is not relevant when talking about crop factor - unless you are assuming you not only change cameras but move forward or back. A photograph taken with a 100mm lens on a DX camera will essentially be identical (other than IQ and DOF) to a photograph taken with a 150mm lens on an FX camera, when both photographs are taken from the same location.

[ATJ]

I should admit that I had the same confusion about the effect of the crop factor some time back so I actually set up an experiment to prove my understanding, and it proved me wrong.

I stuck the camera on a tripod and mounted an 18-70mm lens. I took a shot at 18mm and a shot at 70mm. I then cropped the 18mm shot to cover the same area as the 70mm shot. The photographs were essentially the same (other than DOF and IQ). This is equivalent to the effect of the crop factor (although more dramatic). Of course, after I had done the experiment, it then seemed obvious to me that it could be no other way as the perspective can't change if you don't change the shooting location (as Digitor points out).

[Mr Darcy]

Yep I've been thinking about it overnight. I agree I was wrong. I would like to see the results of the experiment though. If I had an FX camera, I would try it myself. With just a DX camera, I may try Andrew's method

[digitor]

Yep I've been thinking about it overnight. I agree I was wrong. I would like to see the results of the experiment though. If I had an FX camera, I would try it myself. With just a DX camera, I may try Andrew's method

Here's the thread:

http://www.dslrusers.com/viewtopic.php?f=5&t=27299

Cheers

[ATJ]

http://www.dslrusers.com/viewtopic.php?f=5&t=27299

Thanks for that. Saves me trying to find the images. Note that the chair was a good 2-3 metres in front of the quilt.

[Mr Darcy]

Thank you. IT clears the issue for me.

Though I would still like to see the actual experiment done now that FX cameras are becoming more common.