Did you know there are apps that turn your smartphone into a magnifying glass to help you read printed writing? They use the built-in camera on your smart device to scan documents or pages and enlarge the text on-screen. Here are the best magnifying glass apps for Android and iOS devices.

How many stops ND filter for video

An ND filter is nothing more than a semi-transparent piece of glass that gets placed in front of your lens. What makes it special, however, is that it obstructs a precisely controlled fraction of incoming light, and does so uniformly — thereby not altering image contrast or sharpness. The obstruction also aims to be equal across the visible spectrum, thereby not introducing a color cast (although this isn't always the case). This last characteristic also happens to be why it's called a neutral density filter.

Variable ND Filters & Stacking. With these, the strength can be adjusted without having to switch filters, but they're also much more expensive — often even more so than a few separate filters. For more flexibility on a budget, many photographers therefore just stack multiple ND filters when they need to reproduce the light-reducing ability of a single stronger (but unavailable) filter.

However, let's focus on a specific waterfall example. Without a filter, one would use both the smallest aperture and the lowest ISO speed available. With a waterfall under soft daylight, f/22 and ISO100 might yield an exposure of 1/10 second. Unfortunately, not only is this duration insufficient, but the f-stop also had to be increased to the point of reducing sharpness due to diffraction.

Neutral density filters explainedfor photography

The iPhone has a built-in magnifying app called Magnifier; swipe down from the center of your iPhone's home screen and search for Magnifier.

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Step-Up & Step-Down Adaptors. These prevent you from having to purchase a different screw-on filter size for each lens diameter (usually listed as 77 mm, 72 mm, etc.). One could then just purchase an ND filter that fits their largest lens, and use what's called a "step-up adaptor" to have this same ND filer fit on any smaller lenses.

The Android Magnifying Glass app is as simple as its name, with a clean user interface that’s easy to use and a basic feature set that gets the job done but won’t overwhelm the user.

Technical Note: Recall that each "stop" of light-reduction corresponds with a halving of light. A given filter strength therefore passes only 1/2strength of the initial incoming light, where "strength" is the filter strength in stops. For example, a 3-stop ND filter therefore only passes 1/8th the incoming light (since 1/23 = 1/(2*2*2) = 1/8).

Neutral densityfilter vs polarizer

ND filters are specified by their light-reducing ability, where stronger filters appear as darker shades of gray. Some common specifications are summarized below:

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This is one of the easiest magnifier smartphone apps out there, making it ideal if you're an older user who often feels overwhelmed with modern apps and all of their features.

Longer exposure times can achieve a wide variety of artistic effects, including softening the appearance of turbulent water, blurring waves of blowing grass, or emphasizing motion within a crowd of people. For a full discussion of these and other examples, see the tutorial on using shutter speed creatively.

The Cozy Magnifier & Microscope app has the usual magnifier zoom and lighting features one expects, but what makes it stand apart are its contrast and brightness sliders that add an aspect of image editing to the reading experience.

Photo courtesy of alex campos. Note: above example doesn't use an ND filter, but does illustrate a situation where one could potentially help (depending on the camera's max shutter speed).

During daylight, achieving this effect usually requires having a 10-stop or greater ND filter strength, which blocks all but a staggering 1/1000th of incoming light. Try revisiting the exposure settings from some of your past images to see what ND filter strengths would've been needed to achieve multi-second exposures. For example, if a given series of landscapes used a 1/50 second shutter speed at optimal aperture and ISO speed, then these would've required a 10-stop ND filter to extend this exposure time to 20 seconds.

These sliders work much like tools in image editing apps, and their inclusion here means you can adjust the lighting of whatever the camera sees in real time without having to take a photo and open it in a separate image editing app.

Magnifying Glass With Light, or Mag Light as it’s called once installed on your iPhone, boasts an incredibly streamlined display that takes advantage of almost all of the screen’s real estate: It can show as much of what the camera sees as possible.

Filter Color Casts. Some cheaper ND filters can introduce a color cast. Luckily this isn't a major problem with digital cameras, since this can usually be corrected by adjusting the photo's white balance in post-processing. However, in extreme cases, cheaper filters can introduce color casts which don't correlate with white balance settings — making color cast removal much more difficult.

ND filter calculator

Perhaps the biggest problem with averaging is that moving objects aren't always rendered as continuous streaks — no matter how many photos are averaged. Photos containing people, a visible sun, stars or other moving objects are all particularly susceptible.

OpticalDensityfilter

Many other intermediate strengths exist, but high precision typically isn't needed with ND filters. One can often instead adjust the aperture, ISO or shutter speed by one stop without substantially changing the image.

This app also features a reading light that activates your smart device’s built-in flashlight. The light’s brightness can be adjusted via an easy-to-use slider on the left side of the app, while the screen’s brightness can be dimmed or brightened by sliding your fingers left and right.

The app’s controls are a bit on the small side, which may frustrate you if you have large fingers and a small screen, but it’s very easy to use and isn’t too confusing unlike many of the other magnifier apps on Google Play.

In general, better results are achieved when (i) more photos are averaged, and (ii) the exposure time of each photo is long compared to the interval between them. The above example required about 16 averaged shots to adequately mimic an ND filter, in part because each individual exposure was so brief.

Substitutes. Sometimes other filter types can be used in place of an ND filter. Depending on the size of your GND filters, these can sometimes be used as ND filters if the darker region is large enough to cover the entire lens. Alternatively, polarizing filters can function as 2-3 stop ND filters, but only if one also wants the polarized look, and only when this filter is rotated for maximum effect. Some photographers don't purchase 2 or 3-stop ND filters for this reason, and instead opt for stronger varieties (such as a 10-stops).

Although ND filters are primarily used to achieve longer exposures, a less common application is to enable a shallower depth of field in very bright light. For example, most SLR cameras have a maximum shutter speed of 1/4000 second, so a subject in direct sunlight might therefore require an f-stop greater than about f/4.0 (at ISO100). With a 2-stop ND filter, one could reduce this to f/2.0 — yielding a dramatic improvement in background blur and subject isolation.

Variable ND Filter

How long of an exposure does one typically need? For full effect, multi-second exposures are usually a must. These can render clouds as streaks in the sky, blur moving people beyond recognition, or make waves appear as a uniform, low-laying mist. However, this depends on the nature of the motion, the amount of subject magnification and the desired effect. The key is lots of experimentation.

Filter Mounting Systems. Since ND filters have the same effect regardless of how they're shifted or rotated, using a filter mounting system isn't as critical (unlike with GND filters). However, these still have some advantages, including quicker insertion and removal, or freeing up a hand from holding the ND filter in place.

Using an ND filter allows you to address both of these problems, but the extent to which either is improved depends on how you choose to allocate its effect. With a 5-stop ND filter, the same settings would yield a 32X longer exposure time — giving the water a much silkier appearance. Alternatively, one might feel that a 16X (4-stop) exposure time increase is sufficient when weighed against the potential for having a sharper photo, and could instead also decrease the f-stop to f/16.

Even though they might appear gray or even opaque to our eyes, this isn't how your photo will appear; the camera's metering automatically compensates by letting in more light. However, the viewfinder will still appear very dark, so photographers often compose their image prior to placing the filter in front of the lens.

Magnifying Glass is a free Android app that features all of the functionality one wants from a magnifier app. You can use it to zoom in on printed text with up to 10 times magnification, apply filters for easier reading, and activate your tablet or phone’s light when reading in dim light or in the dark.

Combined with the free color filters, this magnifier Android app is a good choice if you find yourself often struggling to read in unusual lighting conditions.

Neutral densityfilter Ophthalmology

While most other magnifying glass apps only provide one way to zoom in on text, Mag Light lets you use the popular pinch gesture for zooming in and out, in addition to a slider on the right side of the screen.

On the other hand, these situations are rare, and can usually benefit more from photographing the subject under dimmer (and likely less harsh) lighting. These situations are also unlikely to require anything stronger than a 2 or 3-stop ND filter.

Neutral density filters can be used to create any combination of (i) a longer exposure time, (ii) a shallower depth of field and (iii) a sharper photograph. Of these three, the first is by far the most common application, so that's where we'll start.

With Magnifying Glass, you can use your Android smartphone or tablet to zoom in on any text the device’s camera can see while activating the light to get a better look when lighting conditions aren’t at their best. There are no bells and whistles to speak of, but for most people, especially more mature users, this is all they need.

Neutral density (ND) filters reduce the amount of light entering the camera, enabling a longer exposure time than otherwise possible. This can emphasize motion, or make an otherwise tumultuous scene appear surreal and quiescent. Alternatively, an ND filter also enables larger apertures, which can produce a shallower depth of field, or achieve a sharper photo. Either way, this is a useful and often under-appreciated filter that deserves a deeper look. ND filters are also one of the easiest filters to use, and their effect cannot be replicated digitally — at least not with a single shot.

Neutral densityfilter chart

Although ND filters cannot be replicated digitally, certain scenes and exposure times can be adequately emulated by employing a technique called image averaging. This works by combining several separate photos in a way that simulates a single, longer exposure:

In the above example, note how the water appears unusual when only two photos were averaged. Note the appearance of a double image from the distant wave crest. This effect is less pronounced with 4 photos, but hasn't been eliminated entirely.

Magnifying Glass + Flashlight is a free app for iOS and Android devices that makes reading small text much easier. Using the device’s camera, the app displays exactly what it sees on the screen and allows you to zoom in and out by sliding your finger up and down.