Confused about crop sensor vs focal length

[Flare]

If you have a EF-S 17-55mm lens on a canon crop sensor does the 1.6 factor still apply? Is it 17mm or 27mm?

Flare

 

[Derrel]

The focal length of 17mm will ALWAYS be 17mm. Always. On crop-body, or FF body. Buuuut--the equivalent field of view of a 17mm lens setting will be, "Like 27mm is on full-frame".

 

[480sparky41 40]

Regardless of the 'comparison', a 50mm lens is a 50mm lens, no matter which sensor you're using. What changes is the apparent field of view between the two. So if you were to take a full-frame and a crop-sensor camera, set them up side-by-side with, say, 50mm lenses on both, and looked through the viewfinders of them, this is what you would see:

Neither camera-lens combination 'enlarges' or 'reduces' the apparent size of the subject. The bridge and the flowers are the exact same size in both VFs. What is different is the size of the focus screens, which is in direct proportion to the size of the respective sensors. This, in turn, changes the field of view (measured in degrees).

 

[Flare]

Ok the focal length is the same but field of view is the difference. So if I shot a 15mm focal length on a crop sensor that would have an equivalent of a 27mm field of view on a full frame sensor. Did I say that right.

 

[480sparky41 40]

Ok the focal length is the same but field of view is the difference. So if I shot a 15mm focal length on a crop sensor that would have an equivalent of a 27mm field of view on a full frame sensor. Did I say that right.

Close... 15 x 1.5 = 22.5 (for Nikon).. 15 x 1.6 = 24 (for Canon). But you got the idea.

 

[Flare]

Sorry bad math but I got. Thank.you for clearing that up.

 

[dbvirago]

Regardless of the 'comparison', a 50mm lens is a 50mm lens, no matter which sensor you're using. What changes is the apparent field of view between the two. So if you were to take a full-frame and a crop-sensor camera, set them up side-by-side with, say, 50mm lenses on both, and looked through the viewfinders of them, this is what you would see:

Neither camera-lens combination 'enlarges' or 'reduces' the apparent size of the subject. The bridge and the flowers are the exact same size in both VFs. What is different is the size of the focus screens, which is in direct proportion to the size of the respective sensors. This, in turn, changes the field of view (measured in degrees).

From someone who just got his first full frame camera yesterday, thanks for this. I've been reading the math for years, but this is the best representation of real life examples I have seen and shows me what I was going to spend part of today discovering. After reading for years about the better 'reach' of a crop sensor, this puts it all to rest.

Thanks

 

[shefjr]

Regardless of the 'comparison', a 50mm lens is a 50mm lens, no matter which sensor you're using. What changes is the apparent field of view between the two. So if you were to take a full-frame and a crop-sensor camera, set them up side-by-side with, say, 50mm lenses on both, and looked through the viewfinders of them, this is what you would see:

Neither camera-lens combination 'enlarges' or 'reduces' the apparent size of the subject. The bridge and the flowers are the exact same size in both VFs. What is different is the size of the focus screens, which is in direct proportion to the size of the respective sensors. This, in turn, changes the field of view (measured in degrees).

This should be a sticky. It's clear concise and to the point.

 

[ratssass]

I understand what you're saying about focal length/field of view,but............why is it when I open my EXIF it's clearly listed as focal length in both apc-s and 35mm equiv.?Semantics?

Focal Length = 24mm
Maker Note = 33202 Byte
User Comment =
Subsec Time = 0.90"
Flashpix Version = Version 1.0
Color Space = sRGB
Exif Image Width = 2448
Exif Image Height = 3696
Sensing Method = One-chip color area sensor
File Source = DSC
Scene Type = A directly photographed image
Custom Rendered = Normal process
Exposure Mode = Manual exposure
White Balance = Auto white balance
Digital Zoom Ratio = 1x
Focal Length In 35mm Film = 36mm

 

[o hey tyler]

I understand what you're saying about focal length/field of view,but............why is it when I open my EXIF it's clearly listed as focal length in both apc-s and 35mm equiv.?Semantics?

Focal Length = 24mm
Maker Note = 33202 Byte
User Comment =
Subsec Time = 0.90"
Flashpix Version = Version 1.0
Color Space = sRGB
Exif Image Width = 2448
Exif Image Height = 3696
Sensing Method = One-chip color area sensor
File Source = DSC
Scene Type = A directly photographed image
Custom Rendered = Normal process
Exposure Mode = Manual exposure
White Balance = Auto white balance
Digital Zoom Ratio = 1x
Focal Length In 35mm Film = 36mm

They put it in there for a 35mm full frame FoV I reckon. I wouldn't say it clearly lists it, as the focal length is up top and the FoV equivalent is way down the list.

 

[ratssass]

my point only being they are both listed as "focal length".Possibly other Exif readers state it differently??

 

[o hey tyler]

my point only being they are both listed as "focal length".Possibly other Exif readers state it differently??

It says focal length, but it's FoV. My exif reader just says "35mm equiv:"

The top one is your focal length. The 35mm equivalent is of no concern to you when viewing exif.

 

[ratssass]

[/QUOTE]

It says focal length, but it's FoV. .[/QUOTE]

...that's the only point i'm trying to make.When a person is learning,something as simple as this lead me to state that I had the focal length of 18-300 covered via 18-105mm (dx),24-70(fx),70-200(fx),when in reality it was FoV.

 

[480sparky41 40]

Here's a real-life side-by-side comparison:

I threw some of my stuff on a table, set my D600 up on a tripod, and mounted two lenses on it: My kit 18-105 DX lens set to 50mm, and my kit 24-85 FX lens set to 50mm. I also set the camera to FX mode so it would record the entire sensor when I use the DX lens. These are SOOC images. The only editing done is to resize them for posting here. The EXIF data will be intact so you can check it yourself.

Results:



What the FX camera saw with the FX lens:

What the FX camera saw with the DX lens:

Notice the items are the exact same size. The only difference is, the DX lens is not designed to cover a full-frame sensor, so it will have the black areas around the edges & corners. The reason for this is a crop sensor is physically smaller (in this case, 24mmx35.9mm for the full-frame sensor, 23.6mm x 15.6mm for the crop). So a DX-format lens projects subjects onto the sensor the same as it's FX-format focal length big brother..... it just can't cover the entire FX sensor. Had I switched the camera to DX mode for the second shot, it would record exactly the same image as what is shown in the second half of this demonstration:


I repeated the experiment swapping my D600 with my D7000, using the same two lenses. This time, the subjects will 'appear' closer because of the crop sensors' narrower FOV. Both images taken with the same two kit lenses, set at 50mm

What the DX camera saw with the DX lens:

What the DX camera saw with the FX lens:

Granted.... there's a minor difference in the FOV in all these, but that's simply due to the margin of error. I have no way of knowing exactly what the focal length of the zoom lens is when I set it. Even though it's set at 50mm, it may be 48.7mm, or 49.2mm, or 51.6mm.......

The reason the DX images appears to be 'closer' or 'larger' is due to the FOV caused by the smaller sensor. The images are all the same pixel size (800x530), so the DX format appears to make a 50mm into a 75mm. But both 50mm lenses project the subjects onto the sensors at the same size.... the crop sensor is just smaller.

 

[dbvirago]

Got my 5D MKIII yesterday and tried some real world examples of my own. Shot a house across the street at 50mm with it and the 40D. At first, when I opened them up in LR, I was astonished at the difference, thinking that the crop did indeed 'zoom' in tighter. Then, with both images side by side, I zoomed in to 100% on a detail. Identical size.