Why is my f/stop not changing? (Pentax)

[MinnieForMe]

I'm sure this is my user error but I have a question. I'm using my 28-75 f2.8 lens. The lesson this week is to photograph on TV mode. I was under the impression that on TV when I change my shutter speed, my F/stop would change automatically. It is staying at f2.8. Is this something I need to change in the camera to allow it to change automatically? At the moment, I'm assuming that f2.8 is the default because of the lens attached.

When I shoot on AV mode, the lens allows me to change the f/stop and it automatically changes the SS so I don't know why it would be any different in another mode.

Thanks for any help.

 

[LPZ_Stitch!]

I'm sure this is my user error but I have a question. I'm using my 28-75 f2.8 lens. The lesson this week is to photograph on TV mode. I was under the impression that on TV when I change my shutter speed, my F/stop would change automatically. It is staying at f2.8. Is this something I need to change in the camera to allow it to change automatically? At the moment, I'm assuming that f2.8 is the default because of the lens attached.

What shutter speeds are your trying to set, and in what lighting conditions?

It could be that you're setting speeds that are too high for anything less than 2.8 (and possibly too high even for 2.8 to get enough light).

You might need to raise the ISO to get the aperture to close down....

Try setting a slow shutter speed in bright sunlight ... something like 1/30. At ISO 100, you should definitely need to close down the aperture to get a decent exposure at a low shutter speed like that....

 

[NateNLogansDad]

What shutter speeds are your trying to set, and in what lighting conditions?

It could be that you're setting speeds that are too high for anything less than 2.8 (and possibly too high even for 2.8 to get enough light).

You might need to raise the ISO to get the aperture to close down....

Try setting a slow shutter speed in bright sunlight ... something like 1/30. At ISO 100, you should definitely need to close down the aperture to get a decent exposure at a low shutter speed like that....

 

[GrillMouster]

The scene that the camera is metering is likely too dark for your combination of shutter speed, aperture, and ISO. As has been stated, keep slowing down the shutter speed and/or increasing the ISO sensitivity. Do you have the lens cap on? If so, then the camera is metering for a pitch black scene regardless of what shutter speed and ISO you're using, and it will try to maximize light-gathering by choosing the largest possible aperture.

 

[MinnieForMe]

Hi;
I'm going to retake the picture to double check my settings. I thought I was following the assignment which had me put my ISO at 1600 and ss at 1/1000.
By the way, I'm photographing running water at the sink to see how changing ss changes the picture.
Thank you.

 

[Groucho]

Also, make sure the aperture ring is set to "A". Of course, if it wasn't, the camera would show "F--" for the aperture, since it wouldn't know what it was - the "A" setting lets the camera control the aperture.

Depending on your model, you can also change your Program Line, which will skew the auto settings in various directions - some of which might be more likely to choose a smaller (higher number) aperture.

It's probably like Stitch said, though. Try pointing it right at a bright light source and you should see the aperture change.

 

[NateNLogansDad]

Any luck?

 

[MinnieForMe]

The kids go back to school tomorrow (Tuesday). I'll have 2.5 hours to myself to try it. Thanks for all the help. I really appreciate it.

 

[GrillMouster]

Hi;
I'm going to retake the picture to double check my settings. I thought I was following the assignment which had me put my ISO at 1600 and ss at 1/1000.
By the way, I'm photographing running water at the sink to see how changing ss changes the picture.
Thank you.

I just noticed that you said you were changing the shutter speed, but you didn't say whether you were making it faster or slower. That's very important to know. If you were making it faster (underexposing), then it stands to reason that the aperture would not have changed because it was already at the maximum aperture, f/2.8. It couldn't get any larger to allow more light. If, on the other hand, you were slowing down the shutter speed, the aperture wouldn't change until the shutter speed was slow enough that f/2.8 was no longer necessary.

To illustrate my point:
From experience, I know that I can shoot in my own home indoors with the following settings: f/2.8, ISO 400, 1/30 sec.. You're shooting at f/2.8, ISO 1600, 1/1000 sec., which is three stops under my own settings. The lighting in your home may different from my own, so that could be correct. However, assuming that your light levels are similar to my own, then you'd need to slow down the shutter speed to 1/125 to expose correctly. As you lower the shutter speed the aperture would remain at f/2.8, because that's the largest possible aperture. Once you drop the shutter speed to 1/60, THEN the aperture would change to f/4, because remaining at f/2.8 at 1/60 would overexpose.

 

[MinnieForMe]

I just noticed that you said you were changing the shutter speed, but you didn't say whether you were making it faster or slower. That's very important to know. If you were making it faster (underexposing), then it stands to reason that the aperture would not have changed because it was already at the maximum aperture, f/2.8. It couldn't get any larger to allow more light. If, on the other hand, you were slowing down the shutter speed, the aperture wouldn't change until the shutter speed was slow enough that f/2.8 was no longer necessary.

To illustrate my point:
From experience, I know that I can shoot in my own home indoors with the following settings: f/2.8, ISO 400, 1/30 sec.. You're shooting at f/2.8, ISO 1600, 1/1000 sec., which is three stops under my own settings. The lighting in your home may different from my own, so that could be correct. However, assuming that your light levels are similar to my own, then you'd need to slow down the shutter speed to 1/125 to expose correctly. As you lower the shutter speed the aperture would remain at f/2.8, because that's the largest possible aperture. Once you drop the shutter speed to 1/60, THEN the aperture would change to f/4, because remaining at f/2.8 at 1/60 would overexpose.

Yes, my lesson is to make the shutter speed faster. So, if I understand properly, my f/stop is at the widest at f2.8 so it can not change (open more) to accomodate the faster shutter speed. For example, if I had a f1/4 lens and I was doing this same lesson, my shutter speed would go to the f1.4 to allow in more light to accommodate me making the shutter speed faster which in turn limits the amount of time light is let in. Am I making sense? So, I have two options. I either need to let more light into my kitchen from another source or have a dark picture because there isn't enough light at f2.8, iso 1600, ss 1/1000 in my kitchen to complete this assignment. I've checked my classmates and they all have dark photos at this setting unless they used other lighting.

 

[NLD]

f/ 2.8, 1/1000, and ISO 1600 sounds like it would probably be underexposed. It's hard to say for sure because we don't know what the light is like where you're doing this, but in my experience, in normal ambient light in my house, that would be VERY underexposed.

So then making the shutter speed faster would just make it MORE underexposed. On top of that, making the shutter speed faster is not likely to show you much difference in terms of the effect you see on the flowing water in the image. Slowing down the shutter speed (if you do it enough) should show a difference.

(You'll need to keep the camera very still for very slow shutter speeds. So depending how slow: tripod, remote shutter release or use of delayed timer.)

In general: slower shutter speeds will give a "creamier" look to flowing water. Faster shutter speeds will "freeze" the water.

 

[GrillMouster]

...So, if I understand properly, my f/stop is at the widest at f2.8 so it can not change (open more) to accomodate the faster shutter speed. For example, if I had a f1/4 lens and I was doing this same lesson, my shutter speed would go to the f1.4 to allow in more light to accommodate me making the shutter speed faster which in turn limits the amount of time light is let in. Am I making sense? So, I have two options. I either need to let more light into my kitchen from another source or have a dark picture because there isn't enough light at f2.8, iso 1600, ss 1/1000 in my kitchen to complete this assignment. I've checked my classmates and they all have dark photos at this setting unless they used other lighting.

Exactly. At the point that you've maxed out the aperture, you can raise the ISO sensitivity and continue raising the shutter speed (until you reach the max ISO setting). If the assignment doesn't specify that it must be an indoor shot, then I suggest you go outdoors and photograph a fountain, an outdoor faucet, a water hose, etc.. More light should allow you to use faster shutter speeds.