APS-C Sensors And Telephoto Lenses

Why do manufacturers state for example; 400mm lens, equivalent to 640mm on a APS-C sensor camera. Seems to me that a 400mm is a 400mm no matter what sensor is used. An APS-C sensor is not going to bring the subject any closer than it would on a full frame sensor. The field of view might appear that way but the subject isn't 1.6 times closer. Am I missing something here?

No, you're spot on. A 400mm will create the same fov as a 640 on a full frame sensor. What it really means is that you're framing shots differently. A bird or something will take up more space on a crop sensor than a ff, making it appear closer. Or something.

They state it that way because it appears that most people have an easier time understanding the incorrect statement and what impact it seems to have to them, than what the actual technical effects really are. That is apparent each time we have a thread that pops up here on a photography forum where most seem to not understand what the sensor size effects are with focal lengths, and think they actually have zoom with crop sensor.

Correct. The focal length in mm is a property of the lens optics independent of the sensor size. What the APS-C sensor does is to crop the image by 1.6x, which reduces the angle and field of view. Lenses designed for APS-C (Canon's EF-S lenses) are designed so that the image size only covers the smaller sensor, which makes the lens smaller and lighter.

And this isn't just a telephoto lens issue, btw...

James Emory wrote in post #12528928
The field of view might appear that way but the subject isn't 1.6 times closer. Am I missing something here?

Huh? If you are 100 feet from the subject, you are 100 feet from the subject no matter what lens you use. That doesn't make sense as a line of reasoning. If I use an actual 600mm lens vs an actual 400mm lens, I am still 100 feet away. If I use a 10mm lens, I am still 100 feet away.

I think you know the answer to this question. When you put a 400mm lens on an APS-C camera, your subject will fill the same portion of the viewfinder and resulting picture, as would a 640mm lens when used on a FF camera. You can argue that this is just cropping in camera, and while that may be true, if you cropped the highest resolution FF cameras to the same FOV, you'd still end up with a grainier 8 MP image instead.

TeamSpeed wrote in post #12529358
They state it that way because it appears that most people have an easier time understanding the incorrect statement and what impact it seems to have to them, than what the actual technical effects really are. That is apparent each time we have a thread that pops up here on a photography forum where most seem to not understand what the sensor size effects are with focal lengths, and think they actually have zoom with crop sensor.

Team Speed, how do you put these neat little links into your posts? Is it easily explained?
"Past Equipment History | My Feedback Results | My Photo Gallery | My Personal Mini-Reviews
For Sale: Focus Genie MicroAdjustment Chart"

You use the same blah notation in your signature definition...

TeamSpeed wrote in post #12529480
You use the same blah notation in your signature definition...

OK, whatever URL is....no biggie, was just curious.

He is saying you post a link in your Signature just like you do in a normal thread. There is a little insert link button in the toolbar above the text box to make posts.

tkbslc wrote in post #12529585
He is saying you post a link in your Signature just like you do in a normal thread. There is a little insert link button in the toolbar above the text box to make posts.

OK thanks, will have to play with it next time I sell something or link Flicker pics, etc.

James Emory wrote in post #12528928
Why do manufacturers state for example; 400mm lens, equivalent to 640mm on a APS-C sensor camera. Seems to me that a 400mm is a 400mm no matter what sensor is used. An APS-C sensor is not going to bring the subject any closer than it would on a full frame sensor. The field of view might appear that way but the subject isn't 1.6 times closer. Am I missing something here?

Yes, you are missing something. You appear to believe that major lens manufacturing companies like Canon, Sigma, Nikon, Zeiss, etc are all so stupid that they think that the focal length of a lens actually changes based on camera sensor size. Since this is of course an intensely absurd belief what you are really doing is revealing your own mental limitations.

All the manufacturers are doing is providing you factually correct information to assist with your purchasing and use decisions. When a 400mm lens is used on a Canon APS-C camera the AOV is equivalent to what you would see if you used a 640mm lens on a 35mm camera. That is in fact the case, and that is all they are saying with their equivalency statement. So the statement is accurate, correct, useful, and does not in any way indicate that the manufacturer actually thinks that the lens focal length is magically changed by your sensor size. The big clue to reveal this fact is that they use some variation of the word "equivalent", and also the fact that anyone actually capable of calculating the focal length of the lens they have built is not possibly going to be so stupid as to think the lens focal length changes with sensor size.

arentol, are you having a bad morning?

Our goal is to help people, not belittle them.

A bit brutal but hey. Tough frontier country up there though. They say it like it is...

True however. The important thing here is that we have a "system" with compatible, but varied, technology and there has to be a reference, in this case 35mm/FF. It's not difficult to understand and as stated above, if you can't grasp why the manufacturers quote the equivalent, maybe it's time for a holiday in Seattle.

JohnB57 wrote in post #12529969
A bit brutal but hey. Tough frontier country up there though. They say it like it is...

True however. The important thing here is that we have a "system" with compatible, but varied, technology and there has to be a reference, in this case 35mm/FF. It's not difficult to understand and as stated above, if you can't grasp why the manufacturers quote the equivalent, maybe it's time for a holiday in Seattle.

I'm failing to see how that's any excuse to be an ******* to someone for no reason.