[toni-a]

I am looking at my rainbow shots from this winter as I managed to grab the rainbows several times.
I tried to have foreground and rainbow in focus using relatively small apertures, but did I have to? 
Didn't try it, but if I am taking rainbow behind tree I can use f2.8 or I have to step down?

[MatjazO]

Depends on the distance to foreground and a lens used.

I reckon quite normal use case would be wide angle with a few meters to foreground. For example, APS-C with 24 mm lens at F 2.8 would be ok from 5m. Focus set to 10m. F 5.6 would increase DOF to 4m to infinity.

Any online DOF calculator will be good to calculate parameters for any scene.

[Brightcolours]

Silly question back: is there any reason to consider f2.8 to begin with?

[toni-a]

It's just an example used, my question is does infinity have to be in focus when shooting a rainbow or it is not an issue ?

[Brightcolours]

You put focus on what you want to be in focus, basically. If there is a prominent 2nd subject, I would focus on that instead of the rainbow. It makes no sense to for instance have an important to the photo person not quite in focus, or that car you want to photograph, or that interesting building. Rainbows don't have a lot of detail, so slightly OOF will not matter.

If it is an entire landscape + rainbow that you want to photograph, focus in the same way as when there is no rainbow... So on the main other subject or the horizon.
No need for really small apertures, f5.6 to f11 would do.

Just to get a feel:

APS-C  24mm, f7.1, focussed on the tree. You do not notice the sky/clouds/sun the be OOF.


FF 20mm, f8, focussed on the cables. You do not see the clouds as being OOF.


Focussed on the trucks/signs. Do you notice the background/clouds to be OOF? No. Same with a rainbow.

[toni-a]

You put focus on what you want to be in focus, basically. If there is a prominent 2nd subject, I would focus on that instead of the rainbow. It makes no sense to for instance have an important to the photo person not quite in focus, or that car you want to photograph, or that interesting building. Rainbows don't have a lot of detail, so slightly OOF will not matter.

If it is an entire landscape + rainbow that you want to photograph, focus in the same way as when there is no rainbow... So on the main other subject or the horizon.
No need for really small apertures, f5.6 to f11 would do.

Just to get a feel:

APS-C  24mm, f7.1, focussed on the tree. You do not notice the sky/clouds/sun the be OOF.


FF 20mm, f8, focussed on the cables. You do not see the clouds as being OOF.


Focussed on the trucks/signs. Do you notice the background/clouds to be OOF? No. Same with a rainbow.

45mm f8  let's suppose I used f2.8 will the rainbow still be in focus

[Brightcolours]

Not in focus, but would still look like a rainbow in the background. f2.8 would have no reason, though.

[wim]

You put focus on what you want to be in focus, basically. If there is a prominent 2nd subject, I would focus on that instead of the rainbow. It makes no sense to for instance have an important to the photo person not quite in focus, or that car you want to photograph, or that interesting building. Rainbows don't have a lot of detail, so slightly OOF will not matter.

If it is an entire landscape + rainbow that you want to photograph, focus in the same way as when there is no rainbow... So on the main other subject or the horizon.
No need for really small apertures, f5.6 to f11 would do.

Just to get a feel:

APS-C  24mm, f7.1, focussed on the tree. You do not notice the sky/clouds/sun the be OOF.


FF 20mm, f8, focussed on the cables. You do not see the clouds as being OOF.


Focussed on the trucks/signs. Do you notice the background/clouds to be OOF? No. Same with a rainbow.

45mm f8  let's suppose I used f2.8 will the rainbow still be in focus

Actually, a rainbow is never in focus, it will never look like it is in focus, as it is caused by atmospheric conditions that continuously change with distance, angle of view looking at it, and continuously changing weather / atmospheric conditions.

So I wouldn't worry too much about it if I were you. You can't get it sharp in focus because it is never sharp itself to begin with.

HTH, kind regards, Wim