Originally posted by nskan Happy Healthy New Year.
I was wondering about the difference between HP with Auto ISO and TAV modes. Why would one use one instead of the other, any scenarios where one is preferable?
Welcome to the forum! Which mode or sets of modes you prefer depends on the conditions you are facing and your interests and goals you have in mind.
With the Av mode, you control the aperture, while the camera sets the shutter speed. This mode was the first ever type of automatic exposure available to SLR cameras, or perhaps any cameras. When I started into photography, the Manual mode with the ISO (ASA) also being self-set, was the only mode. These days, the most obvious reason for choosing the Av mode is being primarily interested in controlling depth-of-field (DOF), meaning how much or little of the rest of the scene behind and in front of your main subject will appear in sharp focus.
With the Tv mode, the most obvious reason for choosing it would be in dealing with moving or action subjects, to freeze that action or to establish some degree of blur to indicate motion. In both cases, if you have the ISO set on "Auto", the camera will set that as well as part of the automation, but in my opinion including this in either case here would intrude upon the control of the factors being sought after.
When choosing the Manual mode, this is when you wish control over both action and DOF, and also when you want an exposure to remain the same based on the main subject of interest, even when moving the camera to follow a subject as that subject moves past changing backgrounds. However, if you want to control both DOF and shutter speed for a subject in a stationary situation, but lighting conditions keep changing, then the TAV mode will allow you to do this, while the camera will change the ISO value to compensate exposure for the changing circumstances. So it it is like having the control offered by the Manual mode as to shutter speed and aperture, but with automation that is needed for some circumstances.
The P mode can be a very convenient option for general shooting. It allows for concentrating on composing, framing and shooting. Especially when lighting can change suddenly, as in passing clouds, or just changing your camera position which can change the lighting on and behind subjects. It allows more flexibility than the Av or Tv modes, so you will not be likely to run into compromised situations where shutter speed drops too low, or aperture too wide, as the lighting or background changes. I use this quite often in the way I operate. I usually like to set my ISO value, according to the overall circumstances, but if one also chooses auto-ISO over a given range, this will allow even broader scope for the P mode- say as the daylight is getting dimmer, the camera will then boost ISO also.
But you can also go into menus to adjust the program line. If you are dealing with a lot of activity under these changing conditions, etc. you can bias for action, so there is a tendency for higher shutter speeds, or if wishing a generally greater DOF, you can bias for smaller apertures (higher numbers). If doing this, just remember to change back to normal afterwards.
With Pentax alone, however, they recognized that there can be a quick change in needs. Suddenly, one might need to get control of shutter speed or of aperture. So they came up with the Pentax Hyper System, allowing just that when using the P mode- you can switch into Tv or into Av operation immediately (Hyper Program) by just using the thumb or finger dial to select your preference, without having to reset your mode dial first. Then just hit the green button to return to full P operation. In Manual mode, you can just use the green button to set a meter-centered exposure immediately (Hyper Manual) without having to do the usual time-consuming twirling of thumb and finger dials while watching the meter indicator to get the exposure on target. Then make any needed change of either shutter speed or aperture from there, but if you first hit the AE-L button to lock that exposure value, as you dial the change you are making in either, the other will follow along to preserve the exposure! Very fast and efficient. Great when spot-metering around various parts of a scene. This is the fastest, most efficient operational system out there. Dpreview tested the Pentax K10D and found this system years ago, deeming it "brilliant".
But it was not new, in fact going way back to the Pentax PZ-1 film camera of the mid 1990's. In the late 1990's, I acquired the upgraded PZ-1p, and became acquainted with this system for the first time. Unlike these days, it came with a very good owner's manual. I have used this feature ever since, too many times to count, with various Pentax camera models. Getting the shot at just the right moment is often the most important factor, so then time is a big issue. I remember, after having the PZ-1p for quite some time, Pentax ran a $100 off rebate sale on their Limited prime lenses. I bought the FA 43mm and the FA 77mm Ltd's. I had them along on a hike with a young friend through a woods and down a long slope to a beach. I had the 77mm on camera, and was licking my chops at getting my first shot ever with that lens, and that it would be a portrait. Handily, we came to a large, flat area of a wooden staircase system half-way down that slope, still in the woods. Sunny, with high thin clouds, lighting was excellent, but of course changing as we walked among the trees, so I had the camera on "P" mode. I said to my young friend, "Hey, let me get a picture of you", so he stopped and I walked ahead on the flat area. I turned and raised the camera for an upper-half portrait, ready to execute Hyper control as needed. His body and expression were relaxed and natural with a characteristic smile, and as I looked through the VF I saw the shutter speed was plentiful and aperture at f/4.5, about right with that FL to render the somewhat distant background somewhat blurred so he stood out from it, but still recognizable since it was woodsy and scenic. So I fired the shot instantly. Just one shot and we were on our way. My first shot with the fabulous FA 77mm!! The prints came back, and I was very pleased. I gave him several copies for his family. etc... and the reaction was special, because the result was special, indeed! He was delighted. They all said it was the best photo ever taken of him, including his graduation studio photos! Had the aperture been less than ideal, I could have changed it instantly with HP and got that shot anyway. If you have to fiddle with a camera, your subject's body language and expression often then becomes more posed and no longer as natural.
A Happy New Year to you too, and to all- may it be far better than 2020!!