Regarding field of view and sensor size

[dsiglin]

This much I know: When you take a 50mm full frame lens and put it on say a Sony APS-C mirrorless you have the field of view equivalent of 75mm lens on full frame.
Here's my question: Those full frame lenses are designed to cast a wider circle of light than the APS-C sensor. This is why you get a narrower field of view. But what about a lens designed for APS-C? The image circle it produces is more focused/smaller than the full frame lens. Would that not mean a 50mm APS-C lens has an actual 50mm field of view? In my mind that makes sense but when I see APS-C lenses advertised they often state "equivalent to 75mm on full frame). What am I missing?

 

[Overread]

Focal length is a property solely of the lens itself and is not measured based upon the angle of view that the lens is capable of rendering or is designed for. As a result 50mm is 50mm - no matter if its a lens made for a crop sensor camera or a large format camera. As a result the angle of view you get based upon the sensor/film size will change even though the focal length is the same.

 

[dsiglin]

Yes, I understand focal length is focal length, hence my use of "equivalent". I'm not actually asking about focal length but about field of view. There are speed boosters which reduce the image circle of a full frame lens so that an APS-C sensor has almost the same field of view (to be technical the degree of view) as if it were a full frame sensor. What I'm asking is what about lenses designed specifically for APS-C sensors. They make a smaller more focused circle of light. It seems like it would be doing the same thing as a speed booster. Or is it that they are designed to just show a smaller field of view?

 

[astroNikon]

what Overread stated plus

on a FF lens the image circle is larger than that of a Crop specific lens ( such as if a Nikon lens has "DX" on it).
It's not a smaller more "focused" ... but basically cut off in it's total size.

So if you focus on a house. The house is the same size in both a FF and crop lens. But the crop image is just less of the surrounding overall image.


this was a link in a previous thread - has good images of the "circle" and the crops of that circle
http://www.scantips.com/lights/cropfactor.html

 

[dsiglin]

It's not a smaller more "focused" ... but basically cut off in it's total size.

That's what I was asking. I started pondering this after reading about focal reducers. I guess it is possible for a lens designer to give a true 50mm field of view with an APS-C lens but it would require larger glass than normal and some kind of reducer.

 

[astroNikon]

It's not a smaller more "focused" ... but basically cut off in it's total size.

That's what I was asking. I started pondering this after reading about focal reducers. I guess it is possible for a lens designer to give a true 50mm field of view with an APS-C lens but it would require larger glass than normal and some kind of reducer.

yeah, but are you going to do that to all your lenses?

I got to a point where I was looking at a UltraWide Nikon lens .. it was close to $1300 ...but a refurb'd d600 FF was also $1300 ....
I just went FF and it resolved other problems too. lol I certainly love the decision now.

Basically, be careful how much money you spend trying to resolve a problem. Another solution could be a lot quicker and better.

 

[runnah]

I feel like everyone is going in different directions.

Focal length of a lens doesn't change depending on the body, the field of view changes. So that is why people say "Oh a 50mm on a crop looks like an 85mm on a full frame. Focal length is constant.

 

[Overread]

Here's my question: Those full frame lenses are designed to cast a wider circle of light than the APS-C sensor. This is why you get a narrower field of view. But what about a lens designed for APS-C? The image circle it produces is more focused/smaller than the full frame lens. Would that not mean a 50mm APS-C lens has an actual 50mm field of view? In my mind that makes sense but when I see APS-C lenses advertised they often state "equivalent to 75mm on full frame). What am I missing?[/FONT][/COLOR]

The crop sensor designed lens still has the same 50mm focal length. Whilst the image circle it generates might be smaller, its still giving you the same field of view. Thus when you compare a fullframe 50mm lens and a crop sensor 50mm lens on the same camera body they'd both give the same fields of view (and if you compared them on a fullframe camera body the corners of the frame from the crop sensor lens might be significantly softer or even darker because its image circle is not necessarily as big or as well defined for those edge areas).

35mm equivalents are mostly stated because the majority of photographers (esp in the film days) use or at least have experience of 35mm so its a base line comparison point people can work with.

 

[dsiglin]

Ok so my confusion has been cleared because I discovered focal reducers actually change the focal length of a lens. I did not know this but it perfectly explains why you get a wider angle of view with them.

Basically, be careful how much money you spend trying to resolve a problem. Another solution could be a lot quicker and better.

I'm not looking to buy a focal reducer. That previous post was me pondering how a lens designer could give a 50mm field of view on an APS-C lens.

 

[Derrel]

Focal length and the size of the image circle are two UTTERLY SEPARATE issues. Not. Even. Related. Two, totally different issues, entirely.

There are 300mm lenses designed to cover a 24x36mm sensor or piece of film, and there are 300mm view camera lenses that are designed to cover 4x5 or 8x10 inch sheet film. And yet, the 300mm lenses designed for 35mm film (aka 24x36 format or "135 format") have image circles that cannot even cover a 6x6 centimeter piece of 120 rollfilm--and yet, they are genuine 300mm focal length lenses.

 

[dsiglin]

The speed booster / focal reducer works as the inverse of a tele-convertor adapter. Decreasing the focal length and increasing the field of view.

 

[robbins.photo]

Focal length and the size of the image circle are two UTTERLY SEPARATE issues. Not. Even. Related. Two, totally different issues, entirely.

Hmm. could you go over that middle part again? I'm afraid you lost me.. lol

 

[KmH]

Would that not mean a 50mm APS-C lens has an actual 50mm field of view?

No.
FWIW - all of Nikon's 50 mm prime lenses are FX lenses.

Here are the specs for the AF-S 50 mm f/1.8G

• Maximum Angle of View (DX-format) - 31°30'
• Maximum Angle of View (FX-format) - 47°

A DX (APS-C) 50 mm prime or a zoom set to 50 mm also delivers a FoV very close to or the same as the 31° 30' DX-format FoV.

 

[Derrel]

Focal length and the size of the image circle are two UTTERLY SEPARATE issues. Not. Even. Related. Two, totally different issues, entirely.

Hmm. could you go over that middle part again? I'm afraid you lost me.. lol

There is the angle of view that a lens exhibits. A wide-angle has a wide angle of view. A telephoto lens has a narrow angle of view.

There is also the image circle's coverage; large-format lenses can literally "cover" a LARGE piece of film with light rays. Lenses designed for APS-C cameras "cover" only a small circular area.

This entire area, of lenses, focal lengths, angles of view, and image circle sizes, is fraught with misunderstanding and misconceptions.

Check this thread for more information, and some numbers. Image Circle Coverage and Angle of View - how to calculate?

I only mentioned this because the OP posed a question and it had some erros in understanding in this part: " Those full frame lenses are designed to cast a wider circle of light than the APS-C sensor. This is why you get a narrower field of view. But what about a lens designed for APS-C? The image circle it produces is more focused/smaller than the full frame lens."

I don't even want to get into this whole issue, but the idea that the image circle affects the angle of view is NOT correct, at all. Not correct at all.

 

[robbins.photo]

Focal length and the size of the image circle are two UTTERLY SEPARATE issues. Not. Even. Related. Two, totally different issues, entirely.

Hmm. could you go over that middle part again? I'm afraid you lost me.. lol

There is the angle of view that a lens exhibits. A wide-angle has a wide angle of view. A telephoto lens has a narrow angle of view.

There is also the image circle's coverage; large-format lenses can literally "cover" a LARGE piece of film with light rays. Lenses designed for APS-C cameras "cover" only a small circular area.

This entire area, of lenses, focal lengths, angles of view, and image circle sizes, is fraught with misunderstanding and misconceptions.

Check this thread for more information, and some numbers. Image Circle Coverage and Angle of View - how to calculate?

I only mentioned this because the OP posed a question and it had some erros in understanding in this part: " Those full frame lenses are designed to cast a wider circle of light than the APS-C sensor. This is why you get a narrower field of view. But what about a lens designed for APS-C? The image circle it produces is more focused/smaller than the full frame lens."

I don't even want to get into this whole issue, but the idea that the image circle affects the angle of view is NOT correct, at all. Not correct at all.

Lol.. Derrel, I avoid these discussions of focal length/field of view deals like the plague myself, but had to make an exception just to yank your chain..