A primer on Aperture, f stops, ISO and EV compensation

Thanks BB,
What a lovely suprise to be favourably quoted by a Mod!
You asked for a description of EV compesation? I'm going to have to start with a brief run down of P, A, and S modes though!

P, or Program Auto is a pretty much fully automated setting, where the camera chooses your Apeture, Shutter Speed and ISO for you. A few things (like focus mode and light metering, and the Auto ISO range) can be set to your preference, but on the whole it's automatic.

A, or Apeture Priority is a semi automatic setting. You can directly set the Apeture that you want to use, and the camera automatically selects an appropriate shutter speed and ISO according to it's metering rules.

S, or Shutter Priority is a semi auomatic setting. You can directly set the Shutter Speed that you want to use, and the camera selects an appropriate apeture and ISO.


Well in order to quantify the relationship between: Apeture, Shutter Speed and ISO, Cameras use a number called the "EV". It only exists in Auto, P, S and A modes.

This is a dimensionless variable which need not be confusing, simply put,
when the EV = 0 the cameras light metering is happy that the scene is correctly exposed. (Or more accuratly, the camera is happy that the area you have asked it to meter; be it spot/centre weighted/integrated etc... is correctly exposed.)

This may be the exposure you want too, but it may be not.
For convenient semi-automated shooting, most people use A mode, as the chosen apeture is fundamental to composition.

Imagine you have set your apeture to f4, you have made a consious decision to do this, because you want a healthy amount of depth of field, but still blur the background slightly, and to take advantage of the lens' optimal sharpness at this apeture, you might be photographing a figure against a landscape. You want the apeture to remain fixed at this value.

Now the camera will meter the light in the scene and choose a shutter speed and an ISO that satisfy it. You can see the result on the cameras LCD screen, and it looks good.

You take the picture, but on reviewing it, you notice that the face of your subject is a slightly underexposed, so you adjust the "EV" to +1 and take the picture again. Perfect.

By doing this, you have instructed the camera to either decrease the shutter speed or increase the ISO, you don't mind which, as you just want the shot taken quickly and for it to be exposed well. If time were no object, everyone would shoot in fully manual mode. As long as the ISO stays within the pre-specified boundary, and the shutter speed is not so long as to allow the subject to blur, it works.

Each unit of EV is equivalent to a multiplication factor of 2 applied to either the ISO or the shutter speed (or an equivavlent combination of the two), in this instance.

If we were shooting in Shutter Priority mode, with a shutter speed set at 1/4000s to capture a hummingbirds wings (for example) and the shot were underexposed, increasing the EV +1 would either widen the apeture from f4 to f2.8 or double the ISO (or an equivalent combination of the two). But the shutter speed would remain fixed.

In P mode, the camera has free reign over the apeture, shutter speed and ISO, so when you increase or decrease EV, any and all of them may change.

Now TTL flash EV is another kettle of fish entirely, and I think I'll let someone else explain that one :smile:

 

This is a great read akulya! Thanks!

How about metering modes? What are the different metering modes and which is best for a given situation?

 

I try not to use words like "best", but I understand what you mean and will try to answer.

A camera is just a tool, that does exactly what it's told. Be it by you, or the original programmers, the trick is sometimes, working out how it has been told to behave.

The default light metering is for the whole scene, It's called ESP, and the icon is a rectangle with a dot, a circle, and a couple of lines inside it. This is the cameras attempt to meter the whole scene, I understand that it works quite well, Olympus generally get praise for things like the auto metering and auto white balance working quite well, I can't really comment, but see no reason to disagree, this is the iAuto default.

Centre Weighted metering (dot with ring around it) sets the exposure according to the light levels around the middle of the screen, I find this the most useful, as I often know what my subject is; and want it to be correctly exposed, if the rest of the scene suffers; well i'll just have to sort it out afterwards in post production, or live with it as it is. Almost always, the subject takes precedence.

The trick to doing this, is to centre your subject, focus and meter, then recompose and shoot. Some people customise their buttons to make this process more efficient, but this is what the AEL (auto exposure lock) AFL (Auto focus lock) button is for, it's your camera, set it up the way you want (but put back it on iAuto before you let someone else use it!).

Spot metering is a more extreme version of centre weighted, it has its uses, but the area that gets metered is very small, if a scene had a vast difference in lighting, you would use spot metering for the bit you wanted.

I have not used "High" and "Low" modes, but they are an attempt by the designers to guess what you are trying to achieve, and they may well work. But I generally dislike trying to juggle too many settings, have found Centre and Spot fine for my needs.

 

If you want more explanation with pictures, I HIGHLY recommend you check out "Understanding Exposure" by Bryan Petersen.

He does a great job of not only explaining the relationship between ISO, aperture, and shutter speed, but how to use the A and S modes to achieve your desired effect.

If you want to control depth of field, use A mode. Set a low number for shallow DOF to blur the background, use a higher number to keep everything in focus. Understand that the smaller aperture will result in longer shutter speeds and potentially motion blur ...

If you want the picture to reflect the motion, use S mode and a higher number for more blur and a smaller number for less.

 

luckypenguin

I was just about to mention that.. but you beat me to it... Personally I don't set auto ISO... but rather set the ISO to a level I am comfortable with, shoot aperture priority and keep an eye on shutter speed.

The old rule of thumb in 35mm film terms was that the slowest shutterspeed you could handhold was 1/focal length... so with a 50mm lens a 1/50th or 1/60th was the minimum you should be looking at before seeking additional support.

the 2x crop factor changes this a bit with four thirds cameras - so the rule of thumb becomes

1/(focal length*2)

Image stabilisation does allow you to bend these rules a bit further, but not by as much as the manufacturers claim in most cases...

truth of the matter is that most peoples photos will be improved by higher shutter speeds. If you think your photos are not as sharp as you would like... try putting the camera on a tripod... think you will be surprised

cheers

 

Good point. I honestly can't remember the last time I set a camera to 'S' mode. It isn't wrong, I just don't do that kind of photography. For me aperture is the key to how I want a photo to look. As long the metered shutter speed is somewhere within the zone for my ability to take a shot handheld and the given subject matter I don't particularly care what it ends up being. Amongst other things I have developed a tendency to stop breathing while I am taking a shot for the sake of keeping the camera steady. I'd rather try to keep the camera steady than use a high ISO value.

 

Thanks Nic and Kevin for clearing up the fact that ISO will only change if you have selected "Auto ISO", I should have been more specific about that.

I would second the opinion that for really sharp shots, there is no substitue for a tripod.

Anyone want to contribute on the topic of TTL flash EV (or flash modes in general?) :smile:

 

Thanks for the posts, moving from a P&S to setting the camera myself and understanding what I'm doing has been a bit of a headache these last few weeks.

I've decided to start with A mode and think it's slowly sinking in. I took my first A mode photos at weekend with the aperture wide open to see what they turned out like. If the photos were too bright, would I be right in thinking if I made the aperture smaller, the photos would be a little darker? Or have I got it all wrong?

 

When you half-press the shutter to focus, check the shutter speed readout. On your E-PL1 the max shutter speed is 1/2000sec. I can't remember exactly what the warning looks like ,but if 1/2000 is displayed and it is blinking or coloured yellow, etc. the shot will be overexposed and you will need to close down the aperture.

 

Maybe my explanation wasn't quite right. The photo showed as having no blown/clipped areas but was too bright in my opinion. Would making the aperture smaller make the photo darker, or would the camera change the shutter speed to expose the photo exactly the same?

 

Even if there are no obvious blown or clipped areas the camera may have sill overexposed due to aperture being too large. This would be the case if there were no large variations in light across the scene you were photographing. Check the exif data on the images you took to see what the selected shutter speed was.

 

I think I've now understood what you're saying.

A bright sunny day, the aperture wide open, the camera selects a fast shutter speed ( 1/1600 ) to cope with the bright light and expose the photo in what it thinks is the right exposure.

So, when I select an aperture size, the shutter speed the camera selects will also give an indication as to how the photo will turn out? In this case the fast shutter speed indicated that really the aperture was too wide and I should have selected a smaller aperture to achieve a better looking photo?

Please correct me if my thoughts aren't quite right. I appreciate the help, I just maybe a little slow to understand what I'm being told at the moment :smile:

 

This is correct. To achieve the correct exposure in any of the non-manual exposure modes, the camera will evaluate the scene and make adjustments to whichever combination of aperture, shutter speed, and ISO that you have given it control over.

e.g. In "A" mode with a manually selected ISO setting, the camera has control over the shutter speed only, and it can adjust this value up to the fastest available shutter speed, being 1/2000sec on an E-PL1.

If the required shutter speed is 1/2000sec or slower the image will be correctly exposed, but if it calculates a shutter speed of, say 1/4000sec, the image will be overexposed because the camera can't physically achieve a shutter speed that fast. If this is the case, the camera will give some kind of visual warning, although I don't exactly how the E-PL1 does this. On most cameras the indicated shutter speed value on the LCD will change colour or flash.

If you wish to use your camera at large apertures in bright light it would be best to buy an ND (neutral density) filter which will reduce light transmission into the lens but still give you the visual effect of the large aperture. These are available as ND2, ND4, and ND8. ND2 reduces light transmission by one half (1/2), ND4 by one half again (1/4), and ND8 half again (1/8).

 

Thanks for taking the time to help me understand this. It's a steep curve but I'm getting there bit by bit. I didn't have a lot of time when I took my first photos, so it was a case of take a few photos and see what I could learn from them.

I realise the more photos I take at different settings, the more I will learn about what works in certain conditions. It's going to take time but I'm determined to keep off full auto settings. The great thing with digital is it doesn't matter how many bad photos I take, and I'm prepared for many of those!