Auto ASA

I started using the Auto ASA features in Aperture priority mode only to find the D750 setting itself to ridiculously high ASA values.

I then set the maximum ASA to 3200 and minimum to 200 ASA with a minimum shutter speed of 1/30. The zoom lens I'm using has image stab. and hand held shots at 1/30 are in focus. In daylight the ASA sits at 200 as I expect and in low light the ASA moves up to the limit with the Auto ASA flashing. That seems to work as I would expect.

Then I popped up the internal flash to find the ASA was still set to 3200. I turned off Auto ASA and set it manually to 200. The short range indoor shot with popup flash was repeated and perfectly exposed without noise.

Can anybody explain what is going on? It is almost as if Auto ASA is being set first for a shot without flash to its highest value, then the flash power is adjusted downwards rather than ASA first. I checked the EXIF data and the flash shot was taken at ASA 3200. Do you all use the Auto ASA feature as I'm thinking it may be best to adjust manually in future.

I shoot in Manual mode with Auto ISO. I select the shutter speed and the aperture I want, and the camera sets the ISO as needed.

voxmagna said:

I started using the Auto ASA features in Aperture priority mode only to find the D750 setting itself to ridiculously high ASA values.

I then set the maximum ASA to 3200 and minimum to 200 ASA with a minimum shutter speed of 1/30. The zoom lens I'm using has image stab. and hand held shots at 1/30 are in focus. In daylight the ASA sits at 200 as I expect and in low light the ASA moves up to the limit with the Auto ASA flashing. That seems to work as I would expect.

Then I popped up the internal flash to find the ASA was still set to 3200. I turned off Auto ASA and set it manually to 200. The short range indoor shot with popup flash was repeated and perfectly exposed without noise.

Can anybody explain what is going on? It is almost as if Auto ASA is being set first for a shot without flash to its highest value, then the flash power is adjusted downwards rather than ASA first. I checked the EXIF data and the flash shot was taken at ASA 3200. Do you all use the Auto ASA feature as I'm thinking it may be best to adjust manually in future.

Click to expand...

When you enable Auto-ISO the camera meters just on the ambient light; the fact you've popped up the built-in flash, at this point, means nothing.

If we assume the ambient light is somewhat dim, the camera meter adapts to this by using a little bit wider aperture, a slower shutter speed or a higher ISO depending on what shooting mode you're in. Since you're in "A"perture priority, the ISO is adjusted to compensate for your chosen aperture. Whatever the combination, however, both flash and camera communicate to achieve accurate exposure.

For this reason, and others, I simply don't use Auto-ISO with flash. You can, but I don't really suggest it.
....

voxmagna said:

Can anybody explain what is going on? It is almost as if Auto ASA is being set first for a shot without flash to its highest value, then the flash power is adjusted downwards rather than ASA first. I checked the EXIF data and the flash shot was taken at ASA 3200. Do you all use the Auto ASA feature as I'm thinking it may be best to adjust manually in future.

Click to expand...

Nikon DSLR have three times drastically changed the way Auto ISO works with flash.

At first, the DSLR models up through the D300 would never increase Auto ISO with flash, because, well, because we were using flash instead. Actually, it could increase ISO, but only when and if it was determined that the flash did not have enough power to do it otherwise.
This way seemed pretty great to me, and is apparently what you expected too.

Then the next many models (D300S until the last 4 or 5 models recently), started setting Auto ISO for the ambient reading, regardless if flash was used or not. That's pretty poor. Flash indoors always sees a very high ISO. If we are going to properly expose ambient with ISO, and use flash for fill, then the flash needs color filters to match the ambient white balance.

Then the last very few models (D800 on), which should include your D750, now will only increase Auto ISO by two stops if an external flash is used (hot shoe flash). That typically means from ISO 100 to ISO 400, but 2 stops maximum regardless (4x)... for a hot shoe flash. This is a pretty good plan for bounce flash.

Except, the internal flash is an exception, and it still gets full ISO increase. I suppose because it is a tiny flash and needs the ISO.

You can turn Auto ISO off. That is the best plan.

This is all speaking of TTL flash. If Manual flash mode, the ISO is NEVER increased on any model in any case. Manual flash cannot respond to ISO changes.

Dont use flash but auto ISO is best IMO in manual,then it only goes as high as you make it go via F stop and shutter speed.

mikew said:

Dont use flash but auto ISO is best IMO in manual,then it only goes as high as you make it go via F stop and shutter speed.

Click to expand...

I've turned off Auto ISO and I'm much happier I can set it with the function wheel. If I use aperture priority and see the shutter speed go down to something longer than the lens or I can stabilize, I will just reduce f stop and increase ISO whilst being aware of more noise. If ISO is too high for the shutter/aperture combination, then I either accept the noise, get a tripod, faster lens or put more light on the scene. That's probably what photography is all about?

voxmagna said:

I've turned off Auto ISO and I'm much happier I can set it with the function wheel. If I use aperture priority and see the shutter speed go down to something longer than the lens or I can stabilize, I will just reduce f stop and increase ISO whilst being aware of more noise. If ISO is too high for the shutter/aperture combination, then I either accept the noise, get a tripod, faster lens or put more light on the scene. That's probably what photography is all about?

Click to expand...

The knack is to set the auto ISO at the max your happy with before noise becomes a issue,which ever way you do it doesn't matter,from experience i have an idea what ISO the camera will choose and control it via aperture or shutter speed.

voxmagna said:

I've turned off Auto ISO and I'm much happier I can set it with the function wheel. If I use aperture priority and see the shutter speed go down to something longer than the lens or I can stabilize, I will just reduce f stop and increase ISO whilst being aware of more noise. If ISO is too high for the shutter/aperture combination, then I either accept the noise, get a tripod, faster lens or put more light on the scene. That's probably what photography is all about?

Click to expand...

Well, I'm glad you've found an exposure method that works for you, but I can't really supprort the statement, "That's probably what photography is all about".
...

I have also found the flash and auto ISO "incompatible" when the in-built flash is used. For external flash there is no such problem. Still I use fixed ISO in general. The only time auto ISO is worth it, is when shooting fast moving objects in constantly varying light, and then you generally cannot use a flash.

The idea of using flash is to get extra light for a decent exposure (and low noise). As the flash synchronises at less than 1/300 sec, unless you are using flash for fill, there is no point in using slower speeds, nor can the flash be used at higher speeds, so there is no rationale for using a higher ISO unless the objects is quite far away and/or the flash has low guide number.