Kingston Photographic Club Dealing with High Dynamic Range Part 2 – Practical Methods No Prizes for composition or interest – but there's a problem here

Kingston Photographic ClubDealing with High Dynamic Range

Part 2 – Practical Methods

No Prizes for composition or interest – but there's a problem here.... the camera has done its best but it's underexposed the foreground and overexposed the sky. A very common problem. We could solve it in (at least) four different ways.....



Method 1:Graduated Neutral Density Filters.

These are a cheap method for balancing the light in different areas of an image.

Unlike human eyes, a camera's sensor only sees light as levels of brightness. A large difference in brightness between sky and foreground is beyond their ability to see and record. This is where graduated neutral-density (ND) filters come in. By using one of these filters, you will be able to bring to the sensor something closer to what you can see with your eyes. Grad ND filters cannot create light but they can help you capture it properly.

Pro landscape photographers often use Grad ND's to darken a bright sky so that both the sky and subject can be properly exposed, bringing those areas into a lower contrast Dynamic Range, thus capturing greater detail.

Use these filters when you need to control the contrast range in your landscape images to avoid blowing out highlights like snow-covered mountains or dramatic skies.



Method 1:Graduated Neutral Density Filters.... continued

How do you know when you need one? With some experience, you get a good idea of when the light range in a scene is beyond the ability of the sensor to record, but one sure way is to use your in-camera spot meter and evaluate the scene. Check the difference between the exposure needed for your foreground and the exposure needed for the background. If there's no difference between the two, then you do not need a filter. If there is, say, a 2-stop difference, then you may need a 2-stop filter (ND-4).

Graduated ND filters are available in "soft step" and "hard step" types. The soft-step filters are suited for scenes with no distinct boundary between light and dark zones. The hard step are used for scenes where sky and foreground areas are distinctly separated, usually at the horizon. Soft-step filters are however the universal choice.


Part 2 – Practical MethodsMethod 1:Graduated Neutral Density Filters.... continued

Filter types...... the 'standard' range consists of ND2, ND4 and ND8 filters, giving one, two or three 'stops' respectively. (giving variable light reduction through the dark areas). Additionally the transitions between dark and light on the filters are divided between 'Soft' – giving a very gentle change, 'Hard' – ideal for distinct horizons and 'Medium' – somewhere in between. The most common setup is to have an ND2, ND4 and ND8 filter, all of Soft or Medium transition.

You could get a glass, screw-in filter for each density – they will give the best clarity and be free of any colour casts. The problem is cost – they are expensive and you would want a set (of perhaps six) for each different lens diameter you have. The alternative is a 'system' of square filters – you just need an adaptor ring for each lens, just one filter holder and a single set of filters.

Using the system, you could put all three into the available slots, giving a total of six stops – or use combinations for other amounts. You could also put other filters in, including a Polarising filter.

While Cokin filters are the cheapest and most common in shops, they sometimes have slight colour casts – they are made in batches of thousands, all cut from a single large sheet so any problems are often in a whole batch. The colour casts can be removed in processing but it can be tedious. Generally accepted as the best square filters are the Lee Filters – quality much more consistent but expensive.



With your camera set to Aperture Priority (the easiest), decide on your aperture – let's say f/8. With the camera's metering in Spot mode, aim your camera at the brightest areas and make a

note of the shutter speed given – if you're using ISO100 it's probably something like 1/1000th. Now aim the camera at the darker areas (foreground) – you will probably find something like

1/125th.... so how many stops apart are the foreground and background?Hands up those who said 3! Correct.So we've learned that a 'good' exposure of the sky (to keep details) would be f/8 at 1/1000 th and

that a 'good' exposure of the foreground (to avoid losing details) would be f/8 at 1/125th.... 3 stops apart. Which is nice – because we can make up that difference with just one of our filters! If the difference were greater, we could cover the difference with a combination of filters or try another method.



Method 1:Graduated Neutral Density Filters.... continued

No Filter

2 Stops (ND4)

3 Stops (ND8)



So, is everybody happy with that? Any Questions?

We decided to go for the ND8 – the 3-stop Graduated ND Filter. So this was the result.....



Method 2:Combining different exposures in a Photo Editor

...

In the first method, we started out by considering the differences in Exposure Values required for different parts of the scene.

We looked at the 'good' exposure for the sky and a 'good' exposure for the foreground and worked out the difference.

The ability for a camera to 'bracket' a number of exposures, to try to ensure at least one 'good' one, has been possible for a number of years. Digital cameras have taken this one step further.

Our cameras' Bracket features are usually capable of giving 3 or 5 exposures, each at a range of 'stops' apart... in most cases 0.3, 0.6, 1, 1.3, 1.6, or 2 stops. Easy to do – just choose how many shots, then how many stops apart each shot should be. We could have exposure ranges of anything from a tiny 0.7 over three shots, up to 8 stops over five shots. Check your camera's manual for more details. Is 8 stops enough for anything? Well, nearly.... but we may still need more!


Part 2 – Practical MethodsMethod 2:Combining different exposures in a Photo Editor

When making our bracket choices, at least one exposure will be the 'standard' or 'median' reading given by the meter. The others are over or under that point.

If we choose 3 exposures, 1 stop apart, we get:One exposure as dictated by the meterOne exposure one stop 'faster' One exposure one stop 'slower'

Depending on whether you have Aperture Priority or Shutter Priority selected, the differences will apply to the shutter speed or the aperture respectively.

In Program and Auto modes, the camera may also alter the ISO – which can confuse the issue to some extent – so is best avoided.

It's worth noting that, for landscape pictures, you will generally require a tripod to work with this method. Bracketing, though, could be used at any time – and it was press photographers that wanted it in the first place, just so they could 'make sure' they had the shot they wanted – and that didn't require a tripod.




As long as we have got our heads around the Bracketing feature, we can expect to see three or five differently exposed images.

They should, between them, have a well-exposed foreground in one and a good sky or background in another – it may well be that others can be discarded, for example if you need to use the images with the highest EV for one part and the image with the lowest EV for another.

If you've still got areas which are not well-exposed in at least one shot you may need to take a second set of images, with some Exposure Compensation applied. It can happen! At a location I was at last year, it turned out that the river in the foreground (in shadow) was about 12 stops darker than the sunlit river bank opposite (and this is why you are better off leaving your camera on the tripod - without moving it – until you are satisfied that you have details in every area of the scene).




1/250th @ f/8 1/60th @ f/8 1/15th @ f/8

Using f/8, I chose a three-exposure bracket, 2 stops apart. We can see that the darkest one is of no use but the 'median' exposure and the light exposure had all I needed.



I've chosen two images, one with a good sky and one with a good foreground. Using PS Elements, I've opened both images in the 'workspace' and then dragged one (with the over-exposed sky) onto the other, giving me two layers. I've then put a Layer Mask on the upper layer. At the moment, you can only see the top layer, as it obscures the lower layer.



I've selected the Layer Mask (just by clicking on it) and, with the default foreground and background colours (black and white) selected, I have chosen to use the Brush tool. The Brush should be fairly big – enough to cover about 10% of the total height – and must be 'soft-edged'. I can then 'paint' on the top layer (the large image in the edit window)..... the more black I apply, the more transparent the layer becomes – showing the layer underneath. So I have removed the washed-out sky and revealed the better-exposed sky beneath.



Just flatten the image (right click on bottom layer) to combine the two layers.



Image completed!


Part 2 – Practical MethodsMethod 3:Combining different exposures in an HDR Program

The same method of photographing the images as in Method 3 can be used.

So that means using the Bracket feature of your camera and choosing how many 'stops' the different exposures should be.

A Tripod is almost essential but at a push a steady hand and something to hold on to can work.

Set Aperture Priority mode. (if they were recorded at different apertures, different parts of the image would be in the 'Depth of Field' area. Keep it constant.

Five exposures does the job best and usually one stop apart is fine (a range of 4 stops)– but you may be trying to record a very large Dynamic Range, and that may need up to two stops apart (a range of 8 stops). In general, the more exposures you have, with the narrowest differences, give a better result.



Once we have our three or five exposures we can decide which ones are completely devoid of detail (too heavily under or over exposed) and remove them.

At this stage, we could process them in the normal way and, if your computer isn't all that powerful, make them lower resolution Jpegs. This will make the job of the HDR software easier.

The most popular HDR software, by far, is Photomatix. Photoshop CS4 and CS5 also have HDR capabilities and there are even some free programs (if you want to work with them – the ones I've seen are not very user-friendly).

Explaining how to use HDR software is beyond the scope of this presentation – it's actually very easy and can be perfected with practice. Essentially, the program takes the files you give it and finds the best detailed areas in each of them, then combines them. At this stage it's not going to be good – it needs a process called Tone Mapping before it can be turned into something that your computer can see.



Final HDR result from those five exposures – it can give strange results but this would be considered fairly 'realistic'. Be careful of 'halos' around objects and strange features in the details.


Part 2 – Practical MethodsMethod 4:If you're really desperate.... replace the sky with a sky from another picture!

If anybody's interested, I'll put that in another presentation..... you have enough to be going on with!

Documents

Kingston Photographic Club Dealing with High Dynamic Range Part 2 – Practical Methods No Prizes for composition or interest – but there's a problem here