Focal lengths and crop sensors

[garry71]

I have a Nikon D3100 with the kit lens, 18-55 which I understand to be the same as 28-80 in 35mm full frame terms.

When I put a 35mm full frame 35-80 zoom on the D3100, and set it to say, 35mm and 50mm, it gives the same view and pictures as the kit lens does at the same settings. There doesn't seem to be any 1.5x multiplication happening, with 35mm lenses on the DX body, as it gives the same view when mounted on a full frame 35mm body.

What's going on, or have I misunderstood something?

Cheers
Garry

 

[HoppyUK]

I have a Nikon D3100 with the kit lens, 18-55 which I understand to be the same as 28-80 in 35mm full frame terms.

When I put a 35mm full frame 35-80 zoom on the D3100, and set it to say, 35mm and 50mm, it gives the same view and pictures as the kit lens does at the same settings. There doesn't seem to be any 1.5x multiplication happening, with 35mm lenses on the DX body, as it gives the same view when mounted on a full frame 35mm body.

What's going on, or have I misunderstood something?

Cheers
Garry

Yes, you have misunderstood something It happens - lots of threads on this. It's the size of the sensor that changes, not the lens.

Your camera has a 'cropped' sensor, ie it is smaller than a frame of 35mm film, which is known as 'full frame'. 35mm refers to the width of the film including the sprocket holes, the actual image area of full frame is 36x24mm, and your sensor is roughly half the area with a linear crop factor of 1.5x (1.5 squared =2.25 is the exact area reduction).

Imagine you fit a 50mm lens on to a full frame camera, and note the field of view. Now put masking tape around the sides of the sensor to leave just half the area in the middle. You have now turned it into a crop format sensor, like your D3100, but the lens hasn't changed, only the field of view as captured by the sensor has changed.

Focal length is focal length, but the field of view (the 'effective focal length' if you like) is what changes. To restore equivalence in field of view, you need to multiply focal length by the crop factor, ie to get the same view on both cameras, the crop format camera must have a 33mm lens to match 50mm on full frame.

 

[Barks]

Nice answer Richard

In layman's terms, to work out your FF/35mm equivalent focal length on your camera (for calculating shutter speeds for example) just take the focal length as it shows up on your lens and times it by 1.5, so 50mm to you is 75mm in FF/35mm terms and so on.

 

[garry71]

Thanks for the replies.
I thought DX would have it's own mm sizes like on compact camera lenses.

 

[Dale_tem]

You can mount a full frame lens on a crop sensor and a crop sensor lens on a full frame (only some of the sensor will be used). It therefore makes sense to use the same focal length on all lenses. 35mm is the same on both lenses, its when its attached to the body the multiplication happens due to the sensor.

You just learn that you multiple by 1.6 for crop sensors. in your EXIF data it normally gives the focal length and often the 35mm equivalent.

 

[mark1616]

Thanks for the replies.
I thought DX would have it's own mm sizes like on compact camera lenses.

You can mount a full frame lens on a crop sensor and a crop sensor lens on a full frame (only some of the sensor will be used). It therefore makes sense to use the same focal length on all lenses. 35mm is the same on both lenses, its when its attached to the body the multiplication happens due to the sensor.

You just learn that you multiple by 1.6 for crop sensors. in your EXIF data it normally gives the focal length and often the 35mm equivalent.

It's actually that focal length is a physical property of a lens, no matter what the use is. Just because it is for a crop camera (or going to other way Medium or Large format) you can't suddenly change physics.

There is the 35mm equivalent that lots of people talk about, especially with compact cameras, as this helps people understand the field of view they are going to see, but that's it.

So if you are using a crop camera and you want a field of view to look like a 50mm lens on a 35mm camera you would use a 33mm lens, but if you were using medium format to get that same 50mm you would be using approx an 80mm lens.

Too many people get bogged down with the equivalent field of view, just learn your camera and what you need to shoot with in certain conditions. I work with full frame APS-H (1.3x crop) and APS-C (1.6x crop on Canon) but don't go calculating what each lens is equivalent to on each body, I just use the tool for the job at the time

 

[HoppyUK]

Thanks for the replies.
I thought DX would have it's own mm sizes like on compact camera lenses.

Compact camera lenses have the actual focal length marked on them, and they're very short, like 6-25mm or something, which would translate to about 30-125mm 'equivalent' in terms of the full-frame field of view. Depending on the actual size of the sensor, but most compacts have a crop factor around 5x.

I agree with what's been written above though, in that the crop factor is something to be aware of when you buy stuff or change cameras, but then forget about. It was originally introduced so that film users could relate more easily in the transition to digital.