ND Filters and Aperture:

ND Filters and how to get the most from your wide aperture lens:

Being a photographer on the Costa del Sol, while having its advantages in terms of nice dry weather, isn’t ideal. The midday sun casts harsh shadows and if you don’t take care when placing your subjects you can end up with very unflattering portraits, lots of shadows, squinting eyes and in general a high-contrast, uneven and generally unflattering light. Your choice of aperture is, in a lot of cases, influenced by the daylight conditions and even with a shutter speed of 1/8000s you may find in certain situations the lowest aperture you can go to without over-exposing your image may be f/3.5 or f/4. You may find shooting against the sun that you the lowest aperture is f/8. This may not suit your artistic style or what you are trying to achieve in your composition and without a suitable ND or Neutral Density Filter that compromise may be unavoidable.

Of course your shutter speed is even more critical if you have to use flash? In this case you are dependent on your cameras maximum flash sync speed which for the majority of cameras is 1/250s. If you go above this speed you are into the high speed sync realm of your speedlight and as a result seriously dropping the power you are getting out of your flash which will have big consequences on how your subject is exposed versus the rest of the scene. At 1/250s shutter speed you may be looking at an aperture of f/11 or more to properly expose your composition on a bright day. This may kill your artistic intentions.

From a hardware perspective, a large number of consumer and semi-professional cameras on the market today have a low ISO limit of 200. That extra stop of light control an ISO100 setting will give you may simply not be available depending on the camera you have. For example, you have a scene composed and your camera settings are: shutter speed 1/8000s, aperture f/4, ISO200. But you want to isolate your subject more (a shallower depth of field) so you want to go to the widest aperture on your expensive zoom lens which is f/2.8. However at f/2.8 you have blinking highlights all over your image indicating it is over exposed. What can you do? You have hit the upper ceiling on your shutter speed and also because the low ISO limit on your camera is ISO200 then you have hit the low limit on that also. Other than closing your aperture you have no other option available to you. You have to compromise on your artistic style and take the photo at f/4.

The same applies with shutter speed, a large number of cameras on the market, including the full-frame Nikon D600 camera have a max shutter speed of 1/4000s. This means the upper limit on your shutter speed is 1EV lower than a camera with a shutter speed of 1/8000s. There may be cases where that 1EV makes all the difference.

So if you have a camera with a low ISO limit of 200 and a max shutter speed of 1/4000s, that means you are 2EV’s more restricted from an aperture perspective than the person with a camera that has ISO100 and 1/8000s on their camera.

A lesser known advantage of an ND filter is that it will allow you avoid having your lens aperture greater than the lens diffraction limit. All lenses have this diffraction limit (usually f/8 to f/11) and once above it (for example f/16) your lens is actually dropping in sharpness. So as a landscape photographer if you are above your lens diffraction limit, you are actually reducing the final quality of your image. These days though it would be a rare situation where you would have hit your high limit on your shutter speed, your low limit on your ISO and still be up at f/16.

Another overlooked advantage of an ND filter is the ability to avoid sensor and lens dirt appearing on your images. Lens/sensor dirt is only perceptible at smaller (f/8 and upwards) apertures. The reason is becuase the light is coming from a fixed angle as opposed to more evenly through a wider aperture. If you have some dirt on your sensor then opening your aperture will make it less noticeable and in the case of bright sunny days, an ND filter will allow you to open up your aperture and avoid sensor dirt issues altogether.

In summary, as photographers we must make compromises on our aperture selection when:

  1. Taking photos on bright days with lots of sunlight.
  2. Your camera shutter speed and ISO are limited (to ISO200 and 1/4000s for example).
  3. Using flash on bright days where we want the most power out of our flash.

ND Neutral Density Filters and ApertureSo how will ND filters allow you get around these compromises? Let’s take the first point. If we have a bright sunlit scene and we want to isolate a certain element within the scene, as previously mentioned you may only be able to go to f/4 at 1/8000s and ISO100 without over exposure (or f/5.6 if your low ISO limit is ISO200). Consider the two images on the right. The top one was taken with a 3 stop ND filter and the bottom was taken without an ND filter. The shutter speed was fixed for both photos at 1/4000s and aperture was varied to demonstrate how even a simple 3 stop ND filter can change the whole look of a photo. To me the bottom image is flat compared to the top. The subject "pops" from the background in the top image and this is something that can really only be achieved (without flash) by using wide apertures and in bright sunlight this will only be possible with ND filters. Also the bottom image has a lot of distracting elements in it which are not in the first.

ND Neutral Density Filters and ApertureIf you want to get the most out of your €2000 f/2.8 zoom or f/1.2 or f/1.4 prime and want to isolate the subject as much as possible then the only way on sunny days is to use an ND filter. As another example, place a x16 (4 EV/ 4 f-stop) ND filter in the front of your lens and you are essentially giving yourself 4 extra EV’s to play with. So if your lowest aperture is f/4 without an ND filter then not only can you go to f/2.8 (1EV) you could actually also go to f/1.4 (3EV difference to f/4) and still have 1EV spare to play with.

Considering the second point above, camera limitations, pretend we have a brand spanking Nikon D600 with an 85mm f/1.4 lens on it. It is a bright and cloudless day and we want to get the most out of our lovely 85mm lens as we all know the bokeh is just gorgeous! Now at max shutter speed of 1/4000s and an extended low ISO of ISO 50 we find we can still only use f/5.6 without over exposing. This is no use for us, we want to see that f/1.4 bokeh in all its glory! So we add a x32 ND filter to give us 5 stops extra to play with. We can now increase our ISO to the cameras base ISO value of 100 (always a good idea to stay out of the extended limits when taking serious photos) which uses 1EV of our 5. We then go from f/5.6, to f/4, to f/2.8, to f/2 and finally to f/1.4 using up the other 4EV’s. So now I have setting of: shutter speed 1/4000s, aperture f/1.4, ISO100 and in full sunlight I am now able to take those creamy shots that the 85mm f/1.4 is so famous for without overexposure! The same process can be applied for cases where our cameras have a base ISO limit of 200 (as opposed to 100), using ND filters allows us to “gain” EV’s that we can use to open up our lens aperture giving us far more freedom to pursue our own artistic style.

ND Neutral Density Filters and ApertureOur final point: daylight flash where we want to maintain depth of field yet where we don’t want to exceed our maximum sync speed on our camera is again solved using ND filters. In this case, we set our shutter speed to 1/250s or the max flash sync speed (which can also be 1/200s on some cameras), we set the ISO to the camera’s base (non-extended) ISO value and we then use our cameras meter to determine the correct aperture. On a bright sunny day without an ND filter, you can be guaranteed this aperture will be up around the f/11 mark. This is no use if we want to isolate our subject from a busy or cluttered background. However attach a x16 ND filter (4 extra EV’s) will allow you use that 70-200mm f/2.8 lens you have wide open at f/2.8 isolating your subject perfectly from that messy background. The image to the left is an example of one such situation where you need to balance subject lighting with ambient sunlight using flash and yet want your subject isolated. This image was taken at 1/250s, ISO100 and f/2.8 with an ND8x filter using off camera flash with FEC +1.0EV to balance the harsh sunlight. Isolating your subject like this would be a lot harder without an ND filter.

One word of warning that a lot of photographers will tell you when using ND filters and flash is that your flash will have to work a lot harder than it would without an ND filter. This is only partially true. Flash exposure in simple terms is a function of aperture and ISO, not shutter speed (once below your max flash sync speed). So take an ND8x filter as an example: if your starting settings without the filter are f/8 and 1/250s, when you mount the filter you have effectively reduced the ambient/flash light reaching your sensor by 3 stops (3 EV for ND8x). Now if you used those 3 EV's to open up your aperture by 3 stops (to f/2.8) then you have cancelled out the 3 stop reduction of the filter by opening up your aperture by a similar 3 stops. So in this case, your flash will need exactly the same power output to give the same exposure as it did without the filter. If however you reduce your aperture by two stops only (to f/4) and reduce your shutter speed by one stop (to 1/125s) then your flash will have to work harder as you have starved it of 1EV because shutter speed has no bearing on flash exposure.

So in summary, ND filters have no effect on your flash power output providing you use the benefits of the ND filter for aperture only. If you are not getting the results with your flash in TTL mode when using an ND filter, you can either increase your flash exposure compensation (FEC) or change the mode of the flash to manual mode and set it accordingly. In some cases you may need to up your shutter speed to avoid blur (above maximum flash sync speed). In this case your flash will inevitably have to work harder and will go through batteries a lot faster so you may need to make sure it has sufficient "rest" so as to avoid overheating. In such situations I sometimes use two speedlights off-camera simultaneously to give me that extra flash power. Combining two flashes means individually they don’t have to work as hard so the recycle time is a lot quicker.

In a nutshell, if you’ve got expensive lenses with wide apertures then ND filters are a must on sunny days. They will allow you get the most from your lenses without having to compromise and can turn an average photo into an excellent photo simply by opening up your lens aperture.

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