50 vs 85 1.8

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Aldone Junior Member • Posts: 45
50 vs 85 1.8

I am not 100% sure how this works, so could someone explain which of these two focal lengths (50 or 85) will produce the most shallow depth of field, assuming the same framing? Of course the depth of field is shallower with the 85 if the two are focused at the same distance, but if I step back to achieve the same framing of the subject (ex. A person) the depth of field increases. Which of the two factors, distance and focal length has the greater effect, or do they balance out?

I have a 50, and I’m debating weather to get an 85 for some portraits.

Thanks for the info

aldone

KCook
KCook Forum Pro • Posts: 15,500
Re: 50 vs 85 1.8

If you have a zoom lens(es) that cover 50 and 85 you can run your own trial using those.

Kelly Cook

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AlbertTheLazy
AlbertTheLazy Veteran Member • Posts: 7,751
Re: 50 vs 85 1.8
6

The same field of view and the same aperture will give you the same depth of field, true. However...

If I remember rightly the degree of blurring in out of focus areas will be greater with the longer lens.

Then again, subject isolation is also controlled by the relative distances of the subject and the background and that can be adversely affected by using a longer lens.

Nothing is ever simple in photography!

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Tom Axford Veteran Member • Posts: 6,035
Re: 50 vs 85 1.8
1

Aldone wrote:

I am not 100% sure how this works, so could someone explain which of these two focal lengths (50 or 85) will produce the most shallow depth of field, assuming the same framing? Of course the depth of field is shallower with the 85 if the two are focused at the same distance, but if I step back to achieve the same framing of the subject (ex. A person) the depth of field increases. Which of the two factors, distance and focal length has the greater effect, or do they balance out?

For the same framing, the depth of field at f/1.8 will always be about the same, no matter what the focal length.

However, I would guess that you are probably also interested in how much background blur you might expect to get.  That is not the same as depth of field.

For a background at infinity, the amount of blurring is proportional to the entrance pupil of the lens, which is the focal length divided by the f-number, i.e. 28mm for 50mm f/1.8  and 47mm for 85mm f/1.8.

So the 85mm lens gives the same depth of field, but much more background blur.

OP Aldone Junior Member • Posts: 45
Re: 50 vs 85 1.8

However, I would guess that you are probably also interested in how much background blur you might expect to get. That is not the same as depth of field.

For a background at infinity, the amount of blurring is proportional to the entrance pupil of the lens, which is the focal length divided by the f-number, i.e. 28mm for 50mm f/1.8 and 47mm for 85mm f/1.8.

So the 85mm lens gives the same depth of field, but much more background blurs

this is exactly the kind of technical info I was hoping to get. Thanks a lot. If you also know any good resources on the topic I’d like to learn more

Dem Bell Regular Member • Posts: 185
Re: 50 vs 85 1.8
2

Tom Axford wrote:

So the 85mm lens gives the same depth of field, but much more background blur.

To clarify, for the same framing, the 85 mm gives the same relative amount of blur but magnifies the background more and shows less of it.

If you look at the background blur produced by the 50 mm and the 85 mm, they will have the same appearance except everything in the 85 mm shot will be 85/50=1.7 times bigger.

Tom Axford Veteran Member • Posts: 6,035
Re: 50 vs 85 1.8

Aldone wrote:

However, I would guess that you are probably also interested in how much background blur you might expect to get. That is not the same as depth of field.

For a background at infinity, the amount of blurring is proportional to the entrance pupil of the lens, which is the focal length divided by the f-number, i.e. 28mm for 50mm f/1.8 and 47mm for 85mm f/1.8.

So the 85mm lens gives the same depth of field, but much more background blurs

this is exactly the kind of technical info I was hoping to get. Thanks a lot. If you also know any good resources on the topic I’d like to learn more

For depth of field, most people use an online resource such as dofmaster.com, which contains calculators for depth of field and hyperfocal distance.

For background blur, I haven't come across any tutorial that explains it well, although there are online simulations that show both DoF and background blur, e.g. dofsimulator.net

The formula to compute the size of the background blur (i.e. the diameter of the blur disc produced by a point source of light in the background) is:

blur diameter = entrance pupil x distance between background and subject / distance between background and entrance pupil.

This blur diameter is measured in the plane of the subject (on which the camera is focussed).

Tom Axford Veteran Member • Posts: 6,035
Re: 50 vs 85 1.8

Dem Bell wrote:

Tom Axford wrote:

So the 85mm lens gives the same depth of field, but much more background blur.

To clarify, for the same framing, the 85 mm gives the same relative amount of blur but magnifies the background more and shows less of it.

If you look at the background blur produced by the 50 mm and the 85 mm, they will have the same appearance except everything in the 85 mm shot will be 85/50=1.7 times bigger.

Correct, everything in the background at infinity is magnified 1.7 times (relative to the background in the 50mm shot), but the subject is the same size of course, so the background blur is 1.7 times greater relative to the subject.

(See my previous post for the mathematical formula)

Dem Bell Regular Member • Posts: 185
Re: 50 vs 85 1.8
1

Tom Axford wrote:

Dem Bell wrote:

Tom Axford wrote:

So the 85mm lens gives the same depth of field, but much more background blur.

To clarify, for the same framing, the 85 mm gives the same relative amount of blur but magnifies the background more and shows less of it.

If you look at the background blur produced by the 50 mm and the 85 mm, they will have the same appearance except everything in the 85 mm shot will be 85/50=1.7 times bigger.

Correct, everything in the background at infinity is magnified 1.7 times (relative to the background in the 50mm shot), but the subject is the same size of course, so the background blur is 1.7 times greater relative to the subject.

(See my previous post for the mathematical formula)

Sure. I just wanted to emphasise to other people reading this that because both the amount of blur and the magnification change by the same factor, the background does not look more or less blurry when we use a longer lens and step back to maintain the same framing on the subject.

Say, we can barely make out the letters on a shop far behind the subject when using the 50mm lens. The letters will still be legible if we use the 85 mm lens at the same framing. To blur the letters (so you can no longer read them) one needs a tighter framing, not a longer lens.

Tim Reidy Productions
Tim Reidy Productions Senior Member • Posts: 3,200
Re: 50 vs 85 1.8

If you have a full frame camera you may want an 85mm lens for portraits, if you have an apsc camera than the 50 might be better.

both will be able to produce blur regardless of the sensor used.

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Tom Axford Veteran Member • Posts: 6,035
Re: 50 vs 85 1.8

Dem Bell wrote:

Say, we can barely make out the letters on a shop far behind the subject when using the 50mm lens. The letters will still be legible if we use the 85 mm lens at the same framing. To blur the letters (so you can no longer read them) one needs a tighter framing, not a longer lens.

Yes,  "tighter framing" is equivalent to focussing on something closer and that puts the background further out of focus.

We are both saying effectively the same thing, but using slightly different points of view to describe what happens.

Dem Bell Regular Member • Posts: 185
Re: 50 vs 85 1.8

Tom Axford wrote:

Dem Bell wrote:

Say, we can barely make out the letters on a shop far behind the subject when using the 50mm lens. The letters will still be legible if we use the 85 mm lens at the same framing. To blur the letters (so you can no longer read them) one needs a tighter framing, not necessarily a longer lens.

Yes, "tighter framing" is equivalent to focussing on something closer and that puts the background further out of focus.

But "focusing on something closer" will put our subject out of focus. Yes, it will also blur the background more, but it is not the photograph we want to take, right?

Correction. In my previous post I missed word "necessarily". One can make framing tighter either by bringing the camera and the subject closer to each other or by changing to a longer focal length.

Tom Axford Veteran Member • Posts: 6,035
Re: 50 vs 85 1.8

Dem Bell wrote:

Tom Axford wrote:

Dem Bell wrote:

Say, we can barely make out the letters on a shop far behind the subject when using the 50mm lens. The letters will still be legible if we use the 85 mm lens at the same framing. To blur the letters (so you can no longer read them) one needs a tighter framing, not necessarily a longer lens.

Yes, "tighter framing" is equivalent to focussing on something closer and that puts the background further out of focus.

But "focusing on something closer" will put our subject out of focus. Yes, it will also blur the background more, but it is not the photograph we want to take, right?

When you say tighter framing, you obviously know that that involves both moving closer to the subject and refocussing closer as well.  If you simply left the focus as it was, the subject would be out of focus when you moved closer.  This is so obvious, that I wonder why it needs saying.  Surely it was obvious what I meant.

To spell it out again to avoid any possible misinterpretation (hopefully!):

1. Tighter framing involves moving closer to the subject and refocussing the camera to account for the new subject distance.

2. Simply refocussing the camera to the closer distance (but not moving it) will have exactly the same effect on the background blur, provided the background is at infinity.  Indeed, it is the refocussing that changes the background blur, not moving the camera.

These two things are equivalent in their effect on the background.  They are obviously not equivalent in their effect on the subject!

I hope that explanation makes things clearer.  Changing the focus is what causes the background blur to change.  Moving the camera but leaving the focus distance unchanged will not change the background blur (for a background at infinity).

Correction. In my previous post I missed word "necessarily". One can make framing tighter either by bringing the camera and the subject closer to each other or by changing to a longer focal length.

beagle1 Veteran Member • Posts: 9,729
Re: 50 vs 85 1.8

Aldone wrote:

I am not 100% sure how this works, so could someone explain which of these two focal lengths (50 or 85) will produce the most shallow depth of field, assuming the same framing? Of course the depth of field is shallower with the 85 if the two are focused at the same distance, but if I step back to achieve the same framing of the subject (ex. A person) the depth of field increases. Which of the two factors, distance and focal length has the greater effect, or do they balance out?

I have a 50, and I’m debating weather to get an 85 for some portraits.

Thanks for the info

aldone

the longer lens will give you a more blurred background     .(slightly)

hotdog321
hotdog321 Forum Pro • Posts: 20,097
Re: 50 vs 85 1.8
1

AlbertTheLazy wrote:

The same field of view and the same aperture will give you the same depth of field, true. However...

If I remember rightly the degree of blurring in out of focus areas will be greater with the longer lens.

Then again, subject isolation is also controlled by the relative distances of the subject and the background and that can be adversely affected by using a longer lens.

Nothing is ever simple in photography!

+!. Exactly. I actually had an extremely boring college assignment proving that you get the same depth of field when using the same aperture on different focal length lenses.

But happily you can pretty much ignore this. When working in the real world you appear to get a shallower depth of field by using a longer lens. Conversely, you appear to get more depth of field with a wide angle lens.

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